Finding an absolute 'worst ever' would be tricky, but 'worst I have ever managed to sit through' certainly has its fair share of contenders.
The attempted CGI continuation of Dominion Tank Police, "Tank SWAT" is a firm favourite of mine; cheap looking cg, dull story, and characters that have virtually nothing in common with their previous versions. Bonus points for taking a much loved franchise and putting any hopes of a new series back for who knows how long.
The second series of Zero no Tsukaima deserves a mention, I think. The first series was sort of watchable as it constantly threatened to become almost good. The second series snatches defeat from the jaws of victory by throwing out anything resembling an interesting plot device in favour of more tits.
The worst thing I can remember seeing though is probably one of those OAVs that the sci-fi channel used to show late at night, Makyu Senjo. Admittedly the lousy animation and cliche plotting probably aren't much worse than many of the other late eighties blood-and-thunder efforts, but what earns this one a special place in my hall of hatred is that after our hapless hero goes super-sayan for reasons which presently escape me, he promptly rapes an innocent bystander. And then the plot just conveniently forgets about it. Call me old fashioned, but I just don't have any great desire to cheer on Mr Casual Rape as subsequently goes off to save some professor and his daughter. Even for the sci-fi channel, that's just not nice.