Ironically for me, it was Machamp. Back in the day, fighting types were laughable at best.. long before Steel/Dark types and Pyschics could pretty much wtfpwn everything with little effort. The likelyhood of choosing a fighting type for anything seemed like a foolish idea. Well, DP comes around, and I wanted to steer clear of my usual suspects.. namely using pokemon I've always used since the begining of time it seems.. so I give this Machop a chance. He was originally designated to be my Rocksmasher, but over time came to be part of my Elite Four team. He took out Cynthia's Miltoic (I still hate that thing) and a good number of other annoyances the elites had mostly because I was adamant about keeping the Cross Chop/Focus Energy/SM Toss going. Worked out well in the long run. He's actually my highest, most dependable Poke I have currently.
Skuntank would be another. I raised her on the fly at the very end of the game for the elite four. I'm rather proud of her because she's a good staller. Vespiquen was no match with toxic and Smokebomb going.