The original RollerCoaster Tycoon is definitely my favourite of the series. Yes, RCT2 and 3 added something to the game, but I feel that they didn't add enough to actually make them good sequels- they seemed more like expansion packs.
The game concept
You have to build a theme park- either to win a scenario or just for fun. You can do this in pretty much any way you want, but there are certain guidelines within scenarios that you should follow in order to be able to actually win the scenario and unlock new parks to build.
Yes, even this game has downloadable content- which is free! I downloaded a few of these and found them pretty awesome- you can even run a park that's a replica of Alton towers, which I find is pretty awesome!
There were two expansion packs for RollerCoaster Tycoon- Added Attractions (Corkscrew Follies in the US) and Loopy Landscapes, which added new scenarios and new scenario objectives. They allowed you to build much cooler rides in the game and that was pretty awesome.
This game was brilliant, a real highlight of the "tycoon" games (alongside Zoo Tycoon and the Sims, this was one of my favourite games as a child). If you see it in a shop or online, get it, seriously!