Definitely Chargers fans. I'm on record as to how pathetic I think they act.
They exist in a very insecure sports bubble, if you will. SD teams have, historically, set the benchmark for sucktitude. So, when a team has even a glimmer of hope, the hype machine in SD (which includes the fans, media and organizations) works itself into some sort of incestuous Mobius Strip of cornholery. The media starts it off with ludicrous proclamations such as: "This is the most talented team in the NFL" or "This is the best pitching staff in baseball", etc. Next, the teams all buy into the media hype and pass it along to the players who, in turn, become complete douchebags. This part can be confirmed by simply watching any Chargers postgame interview session. You'll see what I mean. Finally, the fans realize that the other two factors in this cluster#$@% are saying stuff and they ratchet it up another notch by proclaiming things like: "Phillip Rivers went to the Pro Bowl last year, so he's clearly in the conversation with Tom Brady and Manning" or "We've got 148 Pro Bowlers on our team... How about we just use that as a lame retort in any reasonable discourse taking place on other team's message boards! YAAAAAAY!".
There isn't another fan base I find more insufferable than SD fans.