It depends. Are you talking about wifi-battling pokemon in this generation or main-game pokemon? For maingame pokemon, Pokemon that many people don't (or rarely) use include Steelix, Bastiodon, Rampardos, the Wormadam trio, Pachirisu, Drapion (Some people use it but most are too impatient to level a Skorupi to 40), Dusknoir, and most other Grass and Rock types. I know that from personal experience and friends. For wifi, theres a bunch of UU'd pokemon.