It all depends on the story. You can get breathtaking 3-D environments in films like Brave or Up, but if I were pressed, I'd opt for 2-D, hand-drawn animation. The Iron Giant and 2011's Winnie the Pooh are examples of beautiful art meshed with a well-done story that will trump many of the more shoddy 3-D offerings also available. And of course, who can forget Walt Disney's masterpiece (in my opinion), Bambi. Quite possibly the best combination of beautiful character animation and exquisite background paintings of any film I know.
http://www.npr.org/2015/03/28/395328714/the-chinese-paper-son-who-inspired-the-look-of-disneys-bambi
http://www.npr.org/2015/03/28/395328714/the-chinese-paper-son-who-inspired-the-look-of-disneys-bambi