This is a topic for which you will never get a definitive answer because so many people have different views about what is good and what isn't, and the experiences (like quality of food, number of characters and character interaction) can vary from day to day, which will influence a review if a person hits the restaurant on a good or bad day.
I have literally seen reviews of the food for particular restaurants that say "It was great!" and a couple of months down the road someone else will say that the food at the same restaurant was "inedible." One person will say they only had 3 characters come by the table. Another person will say they met 8 characters.
So, how can you decide? So much of what we all tell you on this forum is subjective and based on personal opinions.
Here's what I think, based on what we know for sure:
Ariel's Grotto has a bit more of an upscale food selection (for dinner) which would probably be good for adults' tastes. It's an all-Princess meal, though, so if you don't want to meet Princesses that's not the place to go.
Storytellers Cafe Critter Breakfast features some characters that may or may not be popular (Terk, sometimes Meeko, Koda and Kenai, as well as prolific character meal stars, Chip and Dale), but the restaurant itself has a more 'adult' kind of vibe. It's calmer and more sedate than other meals. The buffet is very small. The menu items are not much less expensive than the buffet. There is no character dinner. But, people seem to love the food at the character meal. The food always gets high marks. At this meal you can order alcohol too, since it is in a hotel and not in
Disneyland.
The
main reason I would suggest
Goofy's Kitchen as being a good choice for "adults" is because my friends and I have been eating there
without children on every visit (with the exception of one meal this past December), and it has worked for us! Plus, the buffet is bountiful - everyone can find something they like - however, some people seem to like the food and others do not like it at all. The characters can range from 3 to 8 onsite at a time - I've experienced both scenarios! They also serve alcohol if you request it, so if you like to have a drink with your character dinner or breakfast, this is the way to go. Other than the wide variety of food, however, I don't know if everyone else would think that Goofy's is a great choice
for adults.
While
Minnie & Friends has the most characters (anywhere from 8-14, it seems), I'm not sure it is the best choice for adults based on what people say about the overall atmosphere. It might be a little too frantic and chaotic. The food gets very, very mixed reviews - ranging from "great" to "awful." But people do love the sheer number of characters at this meal.
Surf's Up with Mickey gets rave reviews for the character interaction and theme, and apparently it is usually not very crowded. I think you can also order alcohol at this meal, too.