Pikachu. Words cannot express how much this thing annoys me. The creepy smile, the squeaky voice, the fact that it's overhyped despite miserable stats, and the fact that certain "famous" trainers use them despite being completely weak, and completely out of place on the team of a "high-level" trainer (I mean they can have five big, tough, macho looking Pokemon and then a small rodent that tries too hard to be cute and fails. Which one does not belong with the others?). Ever hear of Thunderstones?
Its behavior in the anime is like it has some sort of brain disorder. In essence, Pikachu is a victim of being hyped by the franchise
If Pikachu was just treated as any other Pokemon, a close runner up would be Growlithe/Arcanine, as I am most certainly not a dog person.