From what I can remember, as I like everyone else, has supped just one or two scoops, for me it has to be Tolly Cobbold, also Goodrich Castle from Goodrich in the Forest of Dean takes some beating..for yuck...
I'm in agreement with Brigham600 ref Yank beer but the stuff I had in the TopGun bar in San Diego was truly horrible - can't remember the name (thank God) but it was almost green in colour and very, very unpleasant!
Ansells, M&B Brew XI, Federation .... (basically any of the fizzy brown keg brews from the late 70s, early 80s) or from recent times Boddingtons, John Smiths Smooth... the beers that think they are ice-creams.
Maybe you should consider expanding the poll to include naming and shaming individual pubs and clubs too. Take Joannas for instance - most of the beer was shite in there and stopped going (also because my wife won't let me). I think they relied on the fact that most matlots were already too pissed to notice.