So far I have watched the following animation movies in 3D: A turtle Tale: Sammy's Adventures; Legend of the Guardians; Gnomeo and Juliet; Rio; Despicable Me; Tangled; Open Season; Animals United; Ice Age 3 and I would like to rank them, maybe it will be helpful to someone.
As Demo-Material I choose Sammy's Adventures, when it's about the pop-out effects, which are many and very good throughout the whole movie (the crab scene and the one with the pink jellyfish in the dark are the first which come to mind). But for the best over-all 3D experience (depth, detail, colors, (almost) no crosstalk and ghosting) I play Legend of the Guardians and Open Season. These two, although lacking pop-up effects, I consider to be the best examples for 3D (the slow-motion flying scenes and especially the battle ones in Legend of the Guardians are spectacular).
Climbing down the ladder, I would say that Tangled, Animals United and Despicable Me deliver, in this order, great 3D performances. Although it has great 3D, I would not advice you to buy Animals United, first of all, because it is very very very boring and secondly, it's also not full screen. Better to buy Tangled/Despicable Me, which are fun movies, full-screen and you will enjoy re-watching them over and over again, especially in 3D.
Pretty far behind them, I place Gnomeo and Juliet and behind it, Rio.
At the bottom-end I would place Ice Age which I found to be almost like the 2D version, but veeeeryyy disturbing for my eyes.
Tomorrow I will watch Megamind; curious where will it fit.