Depends on what you're looking for in a pet frog. Horned frogs (pacman) are very laid back frogs, you won't see a whole lot of them unless they're eating. I would definitely not plan on keeping one in a 10G, you could maybe keep a male in that, but you won't know if you've got a male or female until it grows so best bet would be starting off with a 20G long that you know would fit either sex.
African bulls are if anything a little more active than horned frogs, care is pretty similar (there are a lot of caresheets online), but they aren't as colorful as the pacmans.
American bulls are much more aquatic and much more active frogs. I wouldn't keep one in smaller than a 75G. Probably more fun to keep, but as said they're jumpy, and with them being aquatic you'll need to filter them etc.
If you want an awesome display animal and love watching them eat go for a male pyxie. You simply cannot go wrong with a frog that can get 8"+ long and almost the same across. A few other suggestions would be Budgett's frog, cane or rococo toads.
+1 Pacman. I got my nephew an albino one for his birthday awhile back, they are pretty laid back. White's tree frogs are interesting to keep as well and are laid back for an arboreal.