What do you like?
The 10 you can do some small fish like tetras, guppies, etc. Maybe a couple small puffers if you want a slight challenge.
The 40 can be fun with some dwarf gouramis and some schooling fish like; tetras, barbs, rasboras, etc. And then maybe a school of cory cats for the bottom or kuhli loaches. Then something random to help fill in the top level or another school of small fish. You can do the small puffers again if you want and plant it heavy so you can have other fish too.
The 90 would make a creat cichlid tank or maybe a couple discus or so with some other tank mates or even a nice angle fish tank with tank mates. You can go the oscar route if you don't really want any tank mates. I'd say just look at lots of photos here and online or even got to LFS's and see what they have. Get some ideas first. Then research the fish you like and go from there.