dwarf gouramis, kissing gouramis, blue gouramis (and the like), pearl goruamis, honey gouramis, convicts, firemouths, angelfish, severum, festivum, red devils, green terrors, texas, jags, lots of others...depends on your tank size though
It depends on what kind of tank you want. If you want a 5-10 gallon aquarium, then you can get a Dwarf Gourami. If you get a big tank like a 75 gallon tank, you can get an Oscar. If you have a 20, you can get a goldfish. It really depends on the tank you have and what you want.
Get a BIG tank and get your self a nice arowana. Or a fire eel. Don't be fooled by the cute little ones you see in the pet store, I've seen 36" long 6" thick fire eels. The cool thing about raising one that big would be hand feeding them then. Mine would curl up in my hand and eat blood worms out of my fingers. But then I saw the huge ones at the Shedd Aquarium in chicago, came home and gave mine away.
Why would you want to have a single fish anyway? Most fish develop a better personality when kept in groups. Well, besides those loner fish like puffers, bettas, etc.
Yes, the tank size is important. If you want small and interesting how about a tank with a crab? or a tank with a turtle (check other posts on this topic)?