I second those who voted Cowboy Bebop as best Dub. I watched the series subbed and then bought the DVDs and thought to myself as I watched the dub, "What was I missing?" Most Gundam franchises are also palatable to me, and I need to watch the dub of Code Geass to find out (I only have about half and episode of R2 to judge from, but from that, it sounded alright... the jury is still out). Oh, and Trigun. I was happy with both.
And for the worst... Bleach certainly comes up there for me, though the fact that I saw 200 episodes of it subbed before seeing 1 dubbed MAY just color my opinion a bit. Naruto also, but then again, I tend to have a hate-on for Naruto in any form (Haha, "Believe it!"). I'm fortunate that I haven't seen too many horrid dubs though.
I'm one of the people though, who prefers whatever he saw and heard first, even within the same franchise. Gundam Seed, for instance, I first saw on cartoon network in English. To this day, watching the sub seems a little off for me. Conversely, Seed Destiny, which I first saw subbed, I don't really like dubbed. Not sure why...