If I had a 27G to stock and was after something with personality (but certainly not big) I'd either go with shell dwelling cichlids, a pair of dwarf cichlids or some dwarf puffers.
If you want BIG fish, a 27 gallon may not be the best bet for you. Now as far as the pretty and rare part, you could do a lot with a 27 gallon. As far as fish rarity, some are rare in one place but abundant in another. Where are you from?