I focus on versatility above all else. As a result, I used Thunderbolt, Icicle Crash, Grass Knot, and Wild Charge. Thunderbolt for most water types, Wild Charge for Milotics and other high SpDef waters, Grass Knot for rock and ground, and Icicle Crash for dragons. As far as nature goes, I really lucked out with Mild nature (no penalty for attack, boost for special attack).
Either way, as much as I love using Pikachu all the time, Magneton has comparable offensive abilities to Light Ball Pikachu, with much better defensive abilities. I use Pikachu because it is so versatile. Either way, both Pikachu and Magneton stand no chance against the ubiquitous Golem...