OK, my picks are based on going for a real Disney atmosphere, something you can't really get anywhere else. Also, you said it was for a birthday so I'm thinking of places that would really get involved.
1. Prime Time Cafe - hands down I think this would be the most fun for celebrating a birthday. The CM's will really get into it. We celebrated my uncle's birthday there and it was hilarious.
2. Ohana - great food and atmosphere. Plus piles of meat that is brought straight to your table. I don't know what more a couple of 21 year old guys would want.
3. Restaurant Marakesh - I don't know if this will show up on anyone else's list but here's my reasons. First, the food is great. Excellent! And it's not weird. It's really just normal things like lemon chicken etc... Second, but probably most importantly is the belly dancing floor show. You said this was a celebration for a couple of guys. They will like this. There is some sort of Y chromosome belly dancing reaction. You can really get a good view of this reaction if you sit and watch the crowd as they pass by the MoRocca band outside of Morocco. The band has a belly dancer. Little boys will stop to watch. Grown men pushing strollers will bump into people as they stare. The little boys in the strollers also stare. Teenage boys will grab other teenage boys to stop and watch. Old men will lose the rest of their party as they slow down past the band. Trust me, men like belly dancers. Whether they are 8 or 80 and that same belly dancer dances inside the restaurant. Score! You will be their hero.
Some other fun places:
Sci Fi dine in - hilarious previews for real B movies. Fun atmosphere. But the food selection is not that great in my opinion. You really are going more for the atmosphere.
Coral Reef - never had a bad experience. And the view of the aquarium is great from every angle. Very unique way to dine.
Whispering Canyon Cafe - fun times and all you can eat barbecue skillet!
I did not put Le Cellier on the list. I've eaten there several times and it's good but I don't it screams Disney. Sometimes you want to get away from the goofiness and that's great but I think if this is a celebration trip, live it up and do something goofy at the Prime Time Cafe and go get a steak in a nice restaurant at home.
Oh and I'm sure you know this but you can ask for a 6 inch cake on the spot in either chocolate or vanilla, no advance notice necessary. This sounds small but we ordered one for my cousin's 18th birthday and there were 8 adults and it fed us all. Once you've finished the Disney portions for dinner that cake size is perfect. It will definitely feed a few adults. And they personalized it and added a chocolate embelishment, it was nice.