(Didn't read much of the topic, sorry)
I like the features RCT 2 has, such as being able to build in the air when you hold shift and all the new coasters (not including the ones that are just the same as others but with a different car)
RCT 2 however seems to be more focused on making a theme park, while RCT 1 is more focused on... well... rollercoaster stuff? It's kinda hard to describe. I just think the scenarios in RCT 1 were better than in 2, but I like 2's features more. Plus 2 has a scenario creator so you could probably theoretically just re-make all the scenarios from 1.
In fact... it seems like I'm going to have to go grab RCT 2 off of GoG...Can't really speak much for RCT 3 though. Seems like you have a lot more room to goof around though, especially with the water/pool stuff. My brother made a water slide that launches people 50 feet into the air at high speed, but since they end up landing in a pool it's not considered crashing or harmful.
Speaking of that, people die in RCT 1 and 2, but in 3 they simply get injured and need to go to a medical station.
People in RCT3 will also be in groups rather than individuals, and you can create your own groups and watch them do stuff in your park, so that's pretty cool.
EDIT: I seem to remember, at least in the expansions, RCT 3 having lots of weird or insane kinds of rollercoasters. Coasters that split into two cars that go around their own tracks and then converge into one again, coasters that stop and go up an elevator, then are tilted upwards to go down the other end... that kinda stuff.