there's no single one.
as a list of best ones:
Disney: - Lion King (gets extra being best finnish dub ever. the dub is so marvelous that it is hard say which one is better, original or finnish version.)
Dreamworks:
- Road to El Dorado. (excelent story line, pretty good way to show meaning of true friendship.)
- Spririt: Stallion of the Cimarron (in first time it's feels like a "my little pony" kind of movie, but after few times, you will find more than that. the art is just amazing. nice way of combining cgi, aquarels and many other techniques. one of the most beautiful female character introductions ever seen in animation history.)
Warner Brothers: -Iron Giant. (just marvelous.)
21st Century Fox: -Anastasia. (more traditional musical way of animation. nice artwork.)
Studio Ghibli: - Spirited Away (a best of anime. can't be compared to european / american style cartoons, because really different style.)
most likely I forgot something that should have been mentioned. Disney has made quite few good animations, but some of them are generally misjudged, because they have Disney logo. There's clear change in disney's cartoons somewhere in late 80's. Last Traditional Disnay animation was The Rescuers: Down Under, which was cleary targetted to younger watchers. After that, Disney cartoons have had something to everyone.
EDIT: as a definitions, cartoons are divided under several sub-categories, like Featured Films, Shorts, Series, TV-Series, etc... so, I expected that Reverend ment Featured Films with done more techniques than just CGI. CGI is used for featured films quite long already. Pixar did flying sequences in
Rescuers: Down Under as their first involment in disney featured film, which premiered 1990.
for more infomation about cartoons, check out The Big Cartoon Database:
http://www.bcdb.com/