It seems reasonably safe to say that Bumblebee and Starscream are the weakest named fighters of their respective factions. Bee has been stated to be the youngest, most inexperienced of the bots, and is also a scout as opposed to warriors such as Bulkhead and presumably Arcee. Starscream on the other hand, has repeatedly been shown on the losing end of a fight, having had his ass convincingly handed to him by both a tired and outnumbered Bulkhead as well as a severely injured Arcee. Yet, despite them being the weakest warriors on their respective sides, they are the only ones who have killed other named characters. Oh, and I guess Airachnid has a kill too, but Tailgate was an unimportant recolour who shared voice actors with Jack, so he doesn't count.