W/ our kids (DD - age 8 & DS - age 6), we do the following character meals:
Chef Mickey's (breakfast)
Cinderella's Royal Table (lunch)
Crystal Palace (lunch)
Garden Grill (lunch)
Liberty Tree Tavern (dinner)
If we were only doing one character meal, I'd probaby choose Chef Mickey's. As others have said, Chef Mickey's is just the quintessentail Disney meal experience. The traditional characters are there - Mickey, Minnie, Goofy, Pluto, & Donald. The restaurant has a neat atmosphere - the monorail runs right above the restaurant seating area. Plus, there are fun things like napkin-twirling & parades.
We've always done breakfast because I've read mixed reviews of the dinner buffet, and I figure it's harder to mess up breakfast than it is dinner.

Plus, I find that I can tolerate a buffet easier in the morning than I can at the end of the day - when I'm ready to just sit for a while & let the food come to me. (W/ younger kids, you end up getting up & down at a buffet way too many times!)