I don't have a favorite. RCT2 is great with graphs and stuff and is bright. Great staff and lots of small great details, Really like the Six Flags Premade Parks.
I like on RCT1 that you actually get something if you pass a scenario. you wanna getem all.
RCT3 has way better flats, water slides, peep cam and you can make a movie of your RCT3 park.
I don't have a fav. I play a diffrent one each day!