I'm gonna go with Magnemite/Magneton. (w00t for originality!!) I always find trouble with them in the games, and I never really liked their design. And ok, they kinda creeped me out in the anime(I didn't like the noises they made...and they had eyes...so many eyes....)
Magikarp is one of my favorite pokemon. Granted, you totally can't use it in battle, but darn it, you use the whole "first in lineup, then switch out to finish the battle" trick for a day, and bam! Hello, butt-kicking Gyrados. ^_^ But to each his own. And I definitely get sick of running into them while Surfing.