I'm usually open to new and unusual beers, and sometimes I find a winner, but every now and then, I come across something so vile I literally cannot finish it. The worst to date? Xingu Black Beer from Brazil. Pretty label, interesting bouquet, hideous flavor. Not good cold, can't be good warm.
And I'd have to put National Bohemian up there, too. I spent a lot of time in Baltimore when a friend was at Johns Hopkins, and for the first year or two, all he and his friends drank was Natty Bo. They learned quickly why a lot of the locals nicknamed it "Battery Acid." Seriously.