In my first playthrough, probably my Electivire, because it outsped both her Hydreigon and Archeops, the latter of which I was very surprised with (I let my Chandelure be death fodder to Archeops so I could heal it). Bonus points because it had a speed tie against N's Reshiram, which allowed me to 3HKO it with Ice Punch and two Earthquakes.
Though my starter, my Samurott, was probably just as good. Despite my entire team being Level 60+, it was my only answer to Haxorus, because that thing could inflict super-effective damage on everyone else on my team and OHKO them after one Dragon Dance. Besides, I got to use my very first Pokémon to take on the Champion's best, which was also the very last one I KOed to beat the game. =)