Ratchet

Ratchet is the Autobot medical officer so he does very little actual fighting. However, he took part in all three battles against the Decepticons on Earth and kiled more than his fair share of Decepticons, too. He seemed to care more about knocking the torsos off other transformers that he forgot to heal his own men. Hmm.

However, there were very few Autobot casulties during the war and that is partially Ratchet's doing. However, you could also credit that to the fact that Optimus Prime tends to melt everyone to molten slag with his awesome Matrix of Badassedness before anyone can say a cross word.