Jolteon.
First time I fought the Elite 4 in Red, it was my only pokemon under level 50 (I believe it was 46). I got all the way to your rival at the end, and my pokemon just crapped out on me. They all went down quickly, and I was left with a severely under-leveled Jolteon.
So I sent it out, expecting to lose quickly. However, my Jolteon was apparently a speed beast and got first hit on everything. On top of that, my thunderbolt attack got about 5 critical hits in a row, and a paralysis twice. My lowest level pokemon took out the champion all by itself. Ever since then, I've been rocking Jolteon in every game I play.
