I put RCT1 above 2 for its more memorable scenarios. 2 did include the coaster builder and the scenario editor, but otherwise saying "more of the same" is putting it lightly -- it's quite telling that the expansion packs for the first game actually made for a better "sequel" than RCT2 (and don't even get me started on how lazy RCT2's expansions were). If you liked what you saw though, 2 isn't a bad way of getting more.
3, though often glossed over, has its own appeal. If you feel the game really needed a sandbox mode, 3 has you covered. The scenarios are even less memorable though, sadly. They're usually completed after only a couple of minutes. Still, you can't deny that being able to ride a coaster from a first-person perspective is pretty amazing. And even if you find its scenarios lackluster, I believe there is a project that attempts to recreate the scenarios from the first two games and put them in 3D. Seriously, it's breathtaking riding the coasters in Three Monkeys Park. Oh yeah, if you want to try this one, make sure you have a powerful PC; even modern rigs get brought to their knees when you have a big park.