Swoobat. Fast enough to setup on enemies without super effective attacks and it gets STAB moves early, which is nice. Never having used it and having quick-checked its stats, I never really expected it to help enough. Nevertheless, I kept it until Samson (as I was a bit lazy to work on other psychic types, like Metagross, Gallade and Alakazam) and it was there to provide the necessary support I needed, not to mention the problems it caused to Shelly and Florinia.
Another would be Seaking. I was casually fishing when a great in IVs, lightningrod adamant Seaking came to my hook and got captured. Despite its generally sub-par stats, it helped me through the first battles and as the beast it is, it devastated wave after wave of opponents, until I reached Route 1, where it struggled to keep up with the wild Tauros. His EVs are a mess, because it has been fighting pokemon since day 1 and I didn't take the time to invest in berries. I was suprised about his IVs, because it was after a week that I realized we could check them. Here's his picture