Your options for a character buffet in Epcot are Akershus in Norway or Garden Grille. I've eaten at the Garden Grille before and personally thought the food was fine, but nothing to write home about. The character interaction, however, with Chip & Dale, Mickey, and Pluto was INCREDIBLE. If you read the recent reviews on this restraunt, however, it seems that a LOT of people were really unhappy with their food. It is served family style also, so it's not technically a "buffet".
Akershus I've never been to but am thinking about trying it for lunch next trip. At this place you meet the princesses (and they are available for breakfast as well). It is, however, Norwegian food, and if you're not an adventurous eater you probably won't be a fan. I've looked at their menus and breakfast is the most "American" of all the offerings, with lunch seeming more authentic Norwegian and dinner seeming even more authentic. They have a cold bar with various salads and what not, and then you pick a hot entree that they serve to you. The reviews on this place are pretty mixed, but I want to give it a shot.
Teppan Edo is a "dinner and a show" type place, similar to Bennihana.
Based on all the reviews I've read on here, I would NOT recommend Mexico if you're used to authentic Mexican food. It's pretty much agreed all across the board that this place is gross. Nine Dragons has more mixed reviews but I've read a lot of negatives. I recommend looking at the menus for all the places in Epcot and deciding based on the sorts of tastes you like.