i have the same question... which one to buy. they sure milking it...

they even have bundle that includes 1&2 for pretty much the price of one game

https://store.steampowered.com/bundle/39671/RollerCoaster_Tycoon_Double_Pack/

Personally, IMO it is best to buy this and RCT2, then use openrct2 to play both.
So the bundle the other guy linked and openrct2

I know this topic is ancient, but really is difficult. The Deluxe version comes with RCT1 and both expansions

Classic ALSO adds in Rollercoaster Tycoon 2, and as I understand it ALSO has the expansions for RCT1 in it. At $4.99 right now, the classic is a great buy.

Deluxe comes with both expansions for the original, corkscrew follies (Added attractions outside of the US), and loopy landscapes. Each of these packs has extra ride sub-types that are added when you have them. The links to the wiki below offer far more detail as to what types come in each expansion.

https://rct.fandom.com/wiki/Added_Attractions
https://rct.fandom.com/wiki/Loopy_Landscapes

RCT Classic also adds several QoL improvements over the original versions, mainly a fast forward feature. It even brings back booster track pieces that RCT2 removed for some reason, as well as an actual progression structure with scenarios unlocking as you complete earlier ones like in RCT1.

However, Classic plays more like RCT2 than it does an enhanced version of RCT1 - although it does include almost all scenarios from RCT1 and its expansions, it also comes with some of the flaws RCT2 had such as scenarios only allowing you to charge for the park entrance or the rides, with no choice of either or both like in RCT1, and rides are more like their RCT2 counterparts so no steep drops on the Junior Coaster for example.

Also, out of all 89 scenarios in RCT1, 9 of them didn't make the cut:
Loopy Landscapes - Urban Jungle
Real Parks - Alton Towers, Heide Park and Blackpool Pleasure Beach
Extras - Fort Anachronism and Mega Park
DLC - PC Player, PC Gaming World and gameplay

If a complete edition is what you're after, it may be best to get the original versions of both games, then use OpenRCT2 with your RCT2 installation. You can also play all of RCT1's scenarios this way if you link your RCT1 installation to it. Although, as an official remaster of sorts, Classic isn't a bad way to experience these games either.