Even as an RCT2 player, I might have to say RCT3. Here's why:
The roller coaster tracks in RCT3 have many more realistic, though not close enough to real life, track segments and inversions. RCT2's trains are slightly more accurate, and wooden. steel and suspended coaster supports are much prettier and realistic. The water rides in RCT3 are much more accurate, owing to the fact that there is an expansion pack allowing for pools to be built, while RCT2 STAYS a theme park tycoon. The splashes are much more realistic, the waterslides are accurate to a point, and dinghy slides are slightly improved.
RCT3 allows the option to create a zoo. RCT2 is strictly limited to, as I said, theme parks. Lions and other predators can even escape if the right fence material is not used, creating a rampage and focring peeps to flee. The player must then board a helicopter and snipe them with a tranquilzer rifle, though they can also shoot peeps.
RCT3 has a day. night. and day/night option, allowing for large realism. The park will open in the morning, guests will flock around in the park, and at night close, making the player have to perform status checks on all rides and building new attractions. However, there is NO season pass system, meaning the guests will pay for each day, having the money build up very quickly and making the game easy. RCT2 just has the same brightness throughout, unless it rains, and guests will stay in the park for years on end.
RCT3 isn't limited to isometric view, instead being 3d, and the staff members look more like their real life counterparts. However, even though RCT2's guests are less detailed then RCT3 peeps, the peeps are triangular pieces glued together with eyes. Not much realism in that. But they have much smarter AI and pathfinding, even if it is lackluster. The art-deco stalls in both versions are both unpleasing to the eye and unrealistic, but in RCT2 they look better then their 3 counterparts. But the general stall in RCT3, or whatever you call the counter that can sell anything, is both realistic and reliable. I've seen many use it in all areas of their RCT3 park.
If you crash a ride in RCT2, the trains will seperate, fly randomly in all directions with minimal gravity effect, and explode on impact, killing anyone riding and completely unscathing others. In RCT3, the trains will follow the law of gravity and obey physics, and will still explode, but I expect this is to remove the train from the game.
Water and hills, in RCT3, is the high point of rct3, the reason I got Platinum for 9 bucks. The water, especially waterfalls, are sooo realistic. It reflects, it glares, it creates mist, it actually ripples when peeps swim in it. RCT2, its just water at the top, that doesn't need walls to stay in. Hills, they're so smooth or rough, and not gridded. You can create a rugged peak, or rolling hills, it goes so well. RCT2, well, you've got around 8 different positions land can take, so you better get creative.