I have to admit I prefer English Dub, Not because I think its better because even I know its not however I am a very poor reader (as some of you guys might have noticed) and when I am watching anime or Foreign films then I want to be enjoying the film not having to pause it every 10 seconds to finish reading some subtitles :S
However you asked what the worst dubbing was, Look no further than the Dragonball Movies...they were so lazy they couldnt find someone to say "Piccolo" so they went with "The Green Guy"