It's almost impossible to single one out when there are so many wretched American, ahem, *beers*, and their foreign copycats. But, I'll vote for Keystone *Light*, a stroke of redundant moronism in branding. I mean, any swill that markets itself as so bland that it won't give you "bitter beer face" has GOT to make you want to spit it out, and fast.
The truth is, almost every lite beer would qualify.