I can tell you a really great Dub, which is Baccano!, which is much better than the original japanese dub (also more accurate to the setting, NYC and Chicago in the 1920s-1930s). It is the only anime where I recommend people skip the sub altogether.
I can also tell you the worst dub, well butchering, is the southern cross of macross.
Watching anime for years (like close to 25 years) I can tell you dubs were originally extremely awful in the 80s and nineties, something changed in the late nineties though. Lets say the post-evangelion era resulted in better dubs. Now I tend to pick and choose what I watch dubbed, and what I watch subbed. If the show has more of a euro-american flavor, such as Big-O, spice and wolf and Baccano!, etc I tend to prefer the dub. However if it leans more towards a japanese setting, definately the sub.
On top of that the voice actors have increased substancially in quality.