I voted Lilo and Stitch. However, Goofy's is my favorite but I prefer dinner there. I have been to all four and here is my pros/cons of each:
Storytellers: Pros: Low key, lovely atmospere, really good food. YOu can order off the menu if you prefer to not do the buffet. It won't save you money but you won't have to stand in line for your food. Cons: not many characters, if you don't like Brother Bear this one is a waste character wise.
Minnie and Friends: Pros: nice variety of characters, right inside the park which is very convenient, pretty, fun setting. Cons: You can only go when the park is open. If that's 10:00 then you can waste a good chunk of your morning with this one. By doing one of the hotel breakfasts, you can go earlier and still be in the park by 10:00. It can sometimes be very hectic in there, with lots of backup at the food lines.
Goofy's: Pros: Nice assortment of characters, including at least one princess, good food, very lively and fun. Cons: Can be loud and sometimes too hectic. I believe its also the most expensive of them all.
Lilo and Stitch: Pros: Really good food with some unique choices (polynesian themed selections), characters play lots of games, characters are in Hawaiin garb. You can order off the menu on this one if the buffet does not interest you. Cons: Unless you are staying at Paradise Pier it is the least conveniently located of them all, some people are disappointed the characters aren't in their traditional garb. Our last visit there some poor little girl cried and cried because Minnie was not in her polka dot dress!
This is all just based on my experiences. I have done them all multiple times except Storytellers which I only did once. Probably the best bet is to pick the one that has the characters you want to see the most since thats what character meals are really about!