I have a few...
75g either clown loaches or black ghost knife
20g either thread fin rainbows or german blue ram
10g Apisto. Cacatuoides because thats all thats in there! But I just got them yesterday
Hmmm, I have a few species only tanks, but my fav fish in my 75 are three syndontis, they move in a wonderful ballet around the giant piece of driftwood in the middle of the tank.
Fo my 58, I love my knight gobies, although had to move one temporarily as he was really gasping and have no idea why. He is better now. I have a giraffe pleco in that same take that is fun to watch too, and he is so pretty!
In my 29 gallon tank I would have to say that my Otos are my favorite fish. They keep my glass clean so I don't have to clean it, and they get very active when I am done cleaning my tank.