RCT2 was the best of the bunch IMO. Still got my disk somewhere, though IIRC it did something weird with the visuals last time I tried to play it. :/
RCT3 was meh, not horrible etc, but far from being as good as the others. RCT1 was pretty cool, but the extra stuff in the 2nd makes it the better one.

Good games, wasted many a happy hour/day/week on it. ^_^