I played RollerCoaster Tycoon a lot as a little kid and it was and always has been one of my favorite PC games. I've been playing through some RollerCoaster Tycoon 1 scenarios and while I really miss a whole lot of stuff RCT 2 added that makes park construction easier (auto-removing scenery when placing stuff, getting a better preview when placing stuff, etc) it's still great. I hear RCT 1's scenarios are better than 2's and they look more interesting overall. 

I think RollerCoaster Tycoon World could be really cool. At first it looked like it was going to be crappy but after they switched developers it looks like something I might actually buy. I think the focus on user-made content is really good. I don't know if I'll be able to get over missing the aesthetic from RCT1 and 2, though. That's the big thing keeping me from trying RCT 3 a lot, even though that's probably dumb. RCT:World just looks so gorgeous though, that it could be an acceptable replacement.

I'm looking forward to OpenRCT2 and I hope it gets as far as OpenTTD got, online play and all. Aside from that, it would make it possible to port RCT2 to Android and then we can finally have a portable version of RCT that doesn't suck. RCT for 3DS and RCT 4 Mobile both look awful. RCT in my pocket with the original aesthetics would be the best thing. You could play in line at Cedar Point and then have the best theme park experience possible.