RollerCoaster Tycoon: Deluxe on Steam, GOG & CD
If you love the original game and want to play it the good news it still, mostly, runs on Windows 10. It's easy to find on nearly any PC digital store and it's cheap and comes with all the expansions. The only downside is that it doesn't run very well on Windows 10 as the game window only stretches so far and you can't mouse away from the game while playing it as it sort of locks up your PC from anything else. When I close the game my Windows 10 PC is unresponsive for a bit but after a minute things come back and all is well. Even though the original game runs strangely on my PC, for some people it doesn't run at all. Publisher Atari just took the original game and the XP update and packaged it up for release and while it still runs, it's not the best way you can play it.
RollerCoaster Tycoon Classic
Is a modern remastering of the best games in the series RollerCoaster Tycoon, RollerCoaster Tycoon 2 and all of the expansions and ported to PC, Mac and tablets. I haven't played it but it reviews well and seems to be what fans have been wanting for quite sometime. That said, the PC version does use the same code as the Tablet version and as such the menus look at bit odd in that context but it doesn't seem unplayable because of it.
Open RollerCoaster Tycoon 2
Is one of those open source projects that make you happy that awesome people have free time. Meant to be a complete source recreation of RollerCoaster Tycoon 2, ORCT2 is a very well established project that is extremely faithful to the originals. It's a recreation of the sequel but you can point at your install of the original game to play the original park scenarios if you want. If you are looking for an amazing way to play RollerCoaster Tycoon 2 and the original scenarios on Windows, Mac or Linux and you have the original games you need to check this out.
One tip is that if you can find the original RollerCoaster Tycoon 2 demo you can use that as the demo timeboxed you to 1 hour and the open source version does not. That said, getting RollerCoaster Tycoon 2 isn't hard and any version will do.