Avatar was specifically designed to be in 3D. Things aren't supposed to fly out at you, unless you're watching one of those shitty 3D movies at Movie World. There was more depth to it, which was especially awesome with those big, sweeping shots. I'd highly recommend it in 3D, just to enhance the awesomeness of the visuals.
Clash of the Titans just had 3D tacked on at the end, so I've heard that it's not worth watching in 3D because they didn't quite get it right. I haven't seen it, probably won't bother.
Alice in Wonderland... it was OK in 3D, but you're not missing much if you see it in 2D.