Hmm. Good question. If preseason was considered I guess it would be the Lions because of how many times the Bills played them. However, because the games were PS games for the most part and after they let us play in their home last winter, it seems to almost be a sibling rivalry. Cleveland has some of that same feel with me too. When they lost their team, the Bills welcomed them. The Bills might have even had a home game that was Cleveland day (I think)? So, I guess I would say San Diego or KC due to the AFL histories. Bills beat SD twice for AFL title. Bills lose to Chiefs in game before first Superbowl.