Magneton has always been my favorite. For some reason, I always pictured myself with a shiny Magneton. I'm not always a huge fan of the shiny colors, but I love the simple, near monotone scheme of the shiny Magneton. Tri-attack is also probably my favorite move.
Kingler would definitely be a very close second. I fell in love with Kingler after the Pokemon League episode where Krabby/Kingler owns for Ash, culminating in that ridiculous hyper beam of Golbat.