Less standard zombie mutation types the better, the more you have the less spread out your upgraded zombies will be, and low level zombies are trash zombies in any part of the game, choosing the start which gives gold is a good option for your first playthrough till you have a general idea of what you want from your zombie build.

DO NOT buy stat upgrades past Lv. 2~3 (You can leave mutation% at 0 if you'd like also) until you have decked out how you want your initial infection attack to work in the support items.

Once you have gotten your initial infection skill up, begin leveling up your stats however you want. These all give very small boosts for their costs and is just a money sink essentially and should be secondary to the support.

You may want to spend some coin on this early if you see a 50% off sale.

Your ultimate goal money-wise is to get Virulent Chain, so that your initial hit can start you with 0~20 zombies, then get additional transmission vector capped so that you can raise that to 0~72 starting zombies. You may wish to invest in a few points of transmission vector before getting virulent chain just to help you clear maps far easier and get the bonus perfect rewards.
(I'm serious, you can technically get 72 starting infections with this combo)

Afterwards put points into Permeable walls and then if you feel like it go Virus Explosion and Virus Spill just for the sake of adding more power to your initial attack.

Keep 3~4 grenades and 3~4 mines equipped at all time.

Use mines to wipe out ZIT troopers when they begin to spawn and grenades to pick off groups that are out of the way and you have no zombies to spend or cannot reach.

Canister is a god-send and should have 2 in your build for emergencies, wipe out a group of soldiers or police all at once turning them into zombos.

This then leaves you with one slot left to have either the burger or goo. Honestly the goo is fun, toss three down and let the civilians walk into them and be turned while also DoTing enemies, burger isn't too useful in contrast honestly, just pulls some civies to it then transforms them.

First things first, make sure to upgrade your common zombie as much as you can, and keep your mutations at roughly half the level of your commons. This is where the true stat upgrades come from.

Specials levels really doesn't matter too much, they're expensive and just a point sink once you leveled up your primary mutations, they're mostly for their AoE buffs/debuffs except for the damage dealers such as yeti, dracula and alien.

Also on the note of the 'special' infecteds, you will get two for free early on. Freddy & Jason.
They both function as AoE buffs. Keep them in your bar as they're both very useful supports to your other infecteds. If you defeat the Canadian boss you will get Disco Zombie, which is undoubtedly the most powerful infected because of his time-stop ability which disables cooldowns. He is a very, very useful infected. My common tactic is to drop Disco Zombie then initial infection attack, then toss a Freddy down asap in the center of it and then a Jason while time is starting to resume. Your initial zombiers will be super-fast and durable killing machines for a while. My bar is generally Disco Zombie, Something fun such as Dracula if I got him, Freddy and Jason.

PS: Zomblins may not sound like a useful unit, but they can work as good distractions. They home in on coins when they drop, and have a very high dmg resistance so they will eventually start distracting ZIT and other enemies with guns as they're drawn to the fallen coins and soak up the damage while your commons come over slowly to devour them. Probably better to get a more useful unit instead though early on. They also will find hidden stashes of coins in the maps as well, which sometimes have well over 10,000 coins in them surprisingly.

Every time you buy a DNA upgrade the next upgrades will be more expensive, so shuffle and focus on getting the Common Zombie upgrades unlocked first, with the mutation % trait being optional depending on if you want more commons or more specials to spawn. Also pick up the money upgrade as well, it'll help in the long-run.

Make sure to choose wisely which mutations you want to unlock because each mutation will of course lower your overall killing power as you'll need to spend zombies to upgrade them. The Swarmers is a pretty good combo, they grab and hold their prey and spawn in groups of three when they do spawn. Plus you get the Biker which is also a 3 unit spawn and the standard grabber as well. With that combo you'll be having to level up five different mutations though alongside your commons. But that is still a reasonable amount of units to level up. You can try mix and matching different builds to see what you like the most and adjust your mutation % to fit your build style.

My recommendation is to start clearing tier 1 of each map tile and to focus down whenever a research station pops up, if things get too hard on the 40~kill them all sections then feel free to do some tier 2s and tier 3s around to level up your zombies and level up your initial infection.

Research labs generate research very slowly with only one vial, but if you see more than one~ you will want to stock up on 4~5 grenades and mine weapons and smash it with your Freddy & Jason combo and mine-bombing the backup. Once you have the Kill Them Alls' down fairly well, kill the bosses for some additional buffs and specials.

And that is all, you now know how to kill humanity. Good luck, have fun.

PS:
May make the game a bit less fun as you'll just be steamrolling everything, with even a couple of your commons able to wipe out a ZIT squad within the first 15~28 seconds.

PSS:
Do not expect to actually get on the global leaderboard. It is impossible. Even if you perfect every single map with a score modifier you will not break the 700k bracket.

My personal ranking from my previous match which was a 100% perfect playthrough with a point modifier where I even used thieves to grab all the hidden gold gave me a ranking of #3893 on the global leaderboard. Top players very, very clearly used hacks or exploits to reach 14mill+ points.