Heracross. It doesn't actually help him that much. There's only a few worthwhile Physical Moves, but they do nothing to help his type coverage. He's already able to hit about 300 Pokemon with a Super-Effective 120BP STAB attack (Close Combat and Mega Horn have excellent coverage together) and then the usual Stone Edge and Earthquake give him perfect, resistance-free type coverage and the ability to hit a whopping 556 Pokemon with a Super-Effective attack.
3DS Friend Code: (5086-0874-6462) Pokemon Y Safari: Bug Type with Combee, Volbeat, and Heracross