Joe Swanson! He's has kicked Peter, Glen and Cleveland cul, ass all at the same time, took out little people drug dealers (while in his wheelchair!!) Yes, I know he had a little help from Brian, but I feel Joe did most of the work!
I would have a dit Stewie of course hands down, but I think that all changed when... WHO KICK HIS BUTT... none other than Susie Swanson... Joe's little daughter! So, Joe is the over-welling answer to this question!
So far, it seems to be Joe Swanson. Whether on legs ou in a wheel chair, Joe lays a beat-down on just about anyone he faces.
I have a feeling that Lois Griffin may also be in the running for Best Fighter, having beaten & having her way with Peter. She has also defeated Stewie, but couldn't kill him because he's her son. Unfortunately, she doesn't fight as much as the others, so her record doesn't quite hold up.
I would l’amour to see a death match between Lois & a fully restored Joe!