Well, Ninjask has a nice Base 90 Attack, and a phenomenal Base 160 Speed stat. It makes an incredible Sweeper. It has a 600 000 EXP Base (Erratic Experience path). However, be wary of the fact that Erratic Pokemon are extremely hard to train until about Lv.55. However, after Lv.55, the required EXP will never go over 6000, whereas an average EXP-based Pokemon (Such as Pikachu, Vulpix, Meowth, Pachirisu, etc.) will hit 6000 required EXP by about lv.48.
EDIT: It's come to my attention that ALL prehistoric Pokemon, with the exception of Relicanth and Aerodactyl, also have this low base.