Aurora Borealis

This map has a lot of coastline, lots of trees and mountains to work with, plateaus to build on, and two rivers running through the center. The map has a lot of cool detail and looks great. You will be a bit constrained by the geography, but you're not necessarily boxed in to a small area.
https://gtm.steamproxy.vip/sharedfiles/filedetails/?id=758898755

Bay of Plenty

This map is a large bay with a lot of flat open area, but still there are mountains and coastline to work with. Not the most unique map on this list but it has everything you'd need to build a strong city.
https://gtm.steamproxy.vip/sharedfiles/filedetails/?id=2106090246

Fort Lowell

This map is mainly flat, however it has some of the coolest detail I've seen in a map. The map gives you a bit of power and water at the start, a toll booth near the outside of the map to give you some income, and lots of natural resources. Also, the pre-made roads and intersections are great looking, with tons of awesome detail put into the interchanges so they're interesting and aren't just empty space. The only potential downside is the lack of mountains or huge height geography, but still a fantastic map.
https://gtm.steamproxy.vip/sharedfiles/filedetails/?id=2023268364

Monument Islands

This island looks like someone took an island and cut through it with some canals. It has a super unique shape, and each mini island has slightly different terrain. This map is one of the most interesting one's I've seen. It is a bit small compared to some other maps, but overall seems really fun.
https://gtm.steamproxy.vip/sharedfiles/filedetails/?id=834058968

Portage

This map has an ocean on one side, and mountains on the other. There is a river that runs through the map with a a couple inlets, and a river running through one corner. There's a good amount of flat and hilly space, although there still are a couple mountains to deal with. The real selling point of this island is its canal. There is an awesome looking man-made canal running through the center of the map, which opens up lots of awesome creative options.
https://gtm.steamproxy.vip/sharedfiles/filedetails/?id=2460906426&searchtext=

Sunset Falls

This map is sort of a valley surrounded on 3 sides by mountains, with the ocean on the fourth side. There is a river that runs through the middle on both sides creating an island in the middle, although both sides of the river have lots of space to work with. This map is also more structured, in that it has lots of well-built interchanges to work with, and you are given some water/power in your starting tile. You will eventually need to add more, but this gives you extra perks when starting out.
https://gtm.steamproxy.vip/sharedfiles/filedetails/?id=2134834801

Two Brothers Islands

This map consists of two islands next to each other, both with good amounts of flat space to work with. Your starting area is quite flat, although as you expand outwards away from the intersection of the two islands the terrain gets more and more mountainous.
https://gtm.steamproxy.vip/sharedfiles/filedetails/?id=1777624976

7th Island Enhanced

This map has lots of really strong areas to build in. You're surrounded by ocean and natural mountains, and the terrain is flat but not uniform, you will have to deal with uneven geography in places. There's also a few smaller islands spread around from the starting area as you expand.
https://gtm.steamproxy.vip/sharedfiles/filedetails/?id=1371511817

Ambrose Island

This is a smaller map, with lots of dense forest are and farmland. The map is mostly a single island with a river running as a spiral through towards the center, with a mountain near the middle. It has a very interesting shape, and the numerous trees give it a very lush feel.
https://gtm.steamproxy.vip/sharedfiles/filedetails/?id=512843213

Banyak Archipelago

This is a super interesting map. The map is a collection of three larger islands, with numerous small islands scattered around. If that sounds boring, think again. This map is crammed full of interesting roads and areas already built in. There's a ton of pre-made roads and interchanges (not just highways! Lots of arterials as well). There's props and interesting areas crammed everywhere. You might not have a ton of space to go crazy and build freely, but you will end up with a super interesting and unique city.
https://gtm.steamproxy.vip/sharedfiles/filedetails/?id=1991452506

Boulder Rapids

This map is more plain than others, it's a flat coast on the left side and as you move right things get more mountainous. However, that doesn't mean it's a boring map, and you get a bit more continuous space to work with here when compared to other options.
https://gtm.steamproxy.vip/sharedfiles/filedetails/?id=409024545

Den Drommel -Z-

Not the most unique map on this list, but overall a very solid one. It's a coastal map, with lots of forest and farming areas nearby to take advantage of if you have the Industries DLC. There's a lot of coastline to work with, and a couple lakes and rivers to work with. Overall the map is mostly flat, although there are some forested mountains a bit further away from the starting square. The highways on this map seem pretty solid, and while it isn't the most exciting it's hard to find anything wrong with the map.
https://gtm.steamproxy.vip/sharedfiles/filedetails/?id=590130671

Eden Falls

This map has one of the coolest features, in that there is a road curving next to a giant waterfall. Near the waterfall are a lot of plateaus to work with, and between the steps you have flat space to work with. My biggest complaint is that the highway system near the starting square flips sides, from a typical right hand highway (sorry Europeans) to left handed, and then back. This makes building an interchange in the area a huge pain, and to fix the roads you need to buy an additional 3 tiles in order to fix this issue.
https://gtm.steamproxy.vip/sharedfiles/filedetails/?id=412755751

Isla Malaya

A big crescent shaped island cut into smaller pieces by rivers, with a central island in the middle of the crescent shape. It's mostly flat, although there are some hills and mountains to deal with. This map is more of a medium-size, so not quite as big as the others but not as small as some of the other island maps.
https://gtm.steamproxy.vip/sharedfiles/filedetails/?id=542255750

Köln Cologne

This map is re-creation of the actual terrain that Cologne is built on. It's mostly flat, however there are some hills on either side. There's one large river running through the middle of the whole map, and it does have something not many other maps have (a lake). Also the fact that it's a real location adds a bit to the appeal.
https://gtm.steamproxy.vip/sharedfiles/filedetails/?id=406363944

Sunset Harbor

Not to be confused with the DLC, this is a map that does not require the DLC in order to run. However, it does work great with the Sunset Harbor DLC as it contains all types of fish. This map is a huge peninsula, with lots of pockets of water and some rolling hills. There's a small number of cliffs, but all of them are small and there are not many. A map with a lot of good potential.
https://gtm.steamproxy.vip/sharedfiles/filedetails/?id=2034806081

Tenasi River

This map is a giant winding river through a valley. There's hills with plateaus on each side of the river. Overall the layout is super interesting, and if you're trying to avoid giant sections of flat connected terrain (kind of like the midwest) this map would be a great option.
https://gtm.steamproxy.vip/sharedfiles/filedetails/?id=418775081

The Great Plan

If you're trying to build on a large, flat area, this is probably the map for you. It's a huge (about 5x5) flat island, surrounded by the ocean and rivers on each side. On the other side of each river is a flat bank that slopes up into a mountain as you move further away. For a giant flat area, this is probably the most interesting map you can work with.
https://gtm.steamproxy.vip/sharedfiles/filedetails/?id=647828859

Carriacou

Carriacou is a carribean-inspired island. It's got a really great setting, although it seems super difficult to work with. You have some flatter pockets, but overall this map is extremely hilly and you will have to be super careful as you build and zone.
https://gtm.steamproxy.vip/sharedfiles/filedetails/?id=1810692034

Copper Creek

This map is mostly flat plains with a couple of rolling hills. There's a river (I assume Copper Creek) that runs through the map and splits into two towards the middle. There is a crazy amount of farmland to work with here, however this map contains no port connections, so you won't able to build a harbor or anything similar that send ships outside your city.
https://gtm.steamproxy.vip/sharedfiles/filedetails/?id=431174019

Dream Bay

This map is a few islands connected by bridges, with lots of mountains and bays to work with. It has a really cool idea, however compared to some of the other maps on this list I think the map is a bit too busy, you don't have quite the same buildable area as others. You could definitely make this work, just be prepared to plan carefully around the terrain.
https://gtm.steamproxy.vip/sharedfiles/filedetails/?id=2464627090

Lush Expanse

Lush Expanse is half-mountain, half-hilly. There is a lot of steep coastline (and I think only one sandy beach). Additionally, the coastline is a bit jagged, there's not many flat sections to build a waterfront unless you terraformed.
https://gtm.steamproxy.vip/sharedfiles/filedetails/?id=2496060944

Riverdale

This map is mostly flat, with some small hills. There's ocean on one side with a huge bay connected to the ocean. It has a ton of buildable space and there's nothing stopping you from building a giant city. The hills are there but shouldn't be steep enough to stop you from building, the only mountainous sections are towards the edge of the map. There's nothing wrong with this map, I just don't find it as exciting as some of the others.
https://gtm.steamproxy.vip/sharedfiles/filedetails/?id=942310403

Naone Island

This map is based on the real island of Naoshima. It's different sections of flat plains between tall mountains, with tunnels running through the mountain. It could turn into a really interesting city, but you have quite a few negatives to deal with. These being lack of flat space, no rail connection in to the map, and only one highway entrance from the outside.
https://gtm.steamproxy.vip/sharedfiles/filedetails/?id=1874890609

Wish You Were Here...?

This map is really interesting, and not just for the name. It's a small/medium sized island, but the geography is pretty crazy for a cities skylines map. There are little pockets of flat land on the water, but a good portion of the map is giant mountains with small winding mountain roads, tunnels through mountains, and plateaus climbing up a mountainside. This could be a really awesome map, but you will need to be super creative with how you build and zone on account of the terrain.
https://gtm.steamproxy.vip/sharedfiles/filedetails/?id=2060346563

Flatlands

This map doesn't really have any flaws, it's just not super exciting. It is a completely flat map, with two highways making a + shape in the center of the map. There is also one river running through the middle of the map. If you wanted a giant sprawling flat city, this would be a great choice. It has the most flat buildable area of any map I've seen. However, if you can do with a little bit less space I think The Great Plan is a little more visually interesting.
https://gtm.steamproxy.vip/sharedfiles/filedetails/?id=411444925

Heim

The workshop page for this map says "Trondheim", but in game this map shows up for me as "Heim". It has a really rough hilly terrain with a bay, coast on one side, and a river running through it. It's not a terrible map, but in my eyes it suffers from a big lack of buildable space.
https://gtm.steamproxy.vip/sharedfiles/filedetails/?id=877731436

REVI 2

This map is a bit odd, in that I don't actually have an issue with this map. It seems super well made, it has a really solid mix of flat areas with some hills and mountains, and the layout is really cool. However, this map is a pain to get loaded. You need a huge list of modifications, as well as the Industries and Mass transit DLC to get the full map. On top of that, you need to disable and enable mods in a specific way in order for the map to load correctly the first time. The workshop page has instructions for all of this, it's just a bit of a pain when other maps have great layouts for much less work.
https://gtm.steamproxy.vip/sharedfiles/filedetails/?id=2450739485

The maps in this section can vary in quality, but I'll try to give some thoughts on each one. Each of these maps does require a different DLC (or multiple), so be aware of that when using any of these maps.

Ocean Diamond II

Required DLCs: Mass Transit
This map is a giant main island, with a few small islands nearby. The starting tile has a nice roundabout entrance already put in, and there's a lake to use as a water supply. Near the starting tile is a dam producing a lot of power, but you'll need to unlock some tiles before you can make use of this. The main island has a giant flat-ish area, but as you move out there are some hills and mountains to work around.
https://gtm.steamproxy.vip/sharedfiles/filedetails/?id=1982221470