My favorite is definitely Goofy's Kitchen. The character interaction is really great, and I love the variety of characters. I think the buffet at Goofy's is actully pretty good and provides the most options; I'm not a big fan of the food at Ariels/Captain Mickey's.
My favorite time for a character meal is lunchtime. I don't usually want to spend the time for a character breakfast, because that's when I like to be in the park going on all my favorite rides (before the crowds). Dinners also sometimes seem too rushed. But a nice, big, leisurely lunch to kick off our afternoon break fits my style. I just love having lunch at Goofy's Kitchen!
I think you can make a PS 60 days in advance for GK. I would recommend that you call at least 30 days in advance.