While RCT 3 was a fine game, I think Roller Coaster Tycoon 1 (with expansions) is still the best. The 2D-isometric graphics are much more pleasing to my eye than the full-3D graphics used in later games, it has a nicer UI, it's still extremely feature-full, and overall it really stands the test of time.
Furthermore,
GOG just released RollerCoaster Tycoon: Deluxe, with all the expansions, for only $5.99. Direct-download, no disc needed to play it, and it's guaranteed to work with Windows 7. Definitely worth the $6 if you ask me.