All around consistently worst are no surprise: Natural Ice and Milwaukee's Best.
Worst ever had to be a bad batch: A case of Hoegaarden a few years ago that was just plain vile. I've had it a few times since, still not wild about it, but this particular case was awful. We were playing volleyball and any time someone committed a foul they had to drink the beer. We are not the frat boy beer pong types, and this was not a reward. It was THAT bad.