My son and I love all of them, but WOW was Ice Age a good movie. I'm not saying thats my final answer, but it comes close. Finding Nemo was a fun movie too, and it had a good story line.
Beowulf, when i was the trailer I did not realized it was animation. Other cute movies like Finding Nemo, Ice Age, Kung Fu Panda, etc many on the list.
My personal favourite one is KungFu Panda I really like to watch this movie again and again when I get a free time
It is more of an oldie but the best animated movie for me is Final Fantasy Spirits within. It also has a cool background music "No Falter". The only thing i didn't like about that movie was it's ending but over all the movie was great.