Most of the time, I think that the english voice acting in anime is either alright or sucks. Generally, I agree with people who think that the original voice acting is superior to the american dub (although generally I am not so picky that I won't watch the readily available english dub anyway.) For some shows, however, I don't feel that this is the case. Here are a few:

The Baron from The Cat Returns: Voiced by Cary Elwes. Westley himself. He has a simply dashing british-accented voice, that fits so well with the character. I watched the subbed version first, and the original didn't really make the character stand out to me. Then I watched it again in English, and thought, "...wow. The Baron is a badass. And a pimp."

L from Deathnote: Once again, the voice actor added a certain personality to the character that I didn't get from the original japanese counterpart. There was a quirkiness, a silliness, that added a dimension to the character and made him more likable the English version. In the Japanese, he just seemed very serious. (To be fair, I only watched the second volume of the show subbed.)

Sidoh from Deathnote: This guy is fricken' hilarious. ...in the english dub. "I want my notebook!" Another awesome line of his that I can't find is when an episode ends with him saying, "Well, that sucks." He sounds so goofy and whiny. In the original, he just sounds serious to me.

General Septum from Gundam Wing: Now to be honest, I've never actually heard the original japanese voice acting for General Septum. However, I find it hard to believe that it could be any better. The voice actor did such a ridiculous, over the top job, that I can't think it was anything other than an intentional "So bad it's good" job. (I can't find any videos where he has substantial screen time. I do have a recording of his voice on my cell phone, but it's in qcp format, and you can't listen to it unless you have a qcp player. )

I might think of more later. Anyhoo...

Wuv, Yer Mom