Overall, I tend to take dubs as a whole rather than picking out individual performances that I thought were superior to their Japanese counterparts, but I think the Hellsing dub(s) (if you include both the original anime and the newer OVA series) fall into both of these categories. Not that I find anything particularly wrong with the Japanese cast, but I think the combination of the absolutely stellar cast they assembled for the dub, combined with the fact that it just seems more appropriate hearing the English accents in a series that takes place in England, makes for a more enjoyable aural experience than the original Japanese.

I also have to agree with smurphy above me for pointing out the great work Richard Epcar did in his role as Batou in all of the assorted Ghost in the Shell movies/series. I love his work as both a director and a voice actor, and I think he just makes the character of Batou, who happens to be my favourite character in that franchise.

On the more general note, and as I was getting at earlier, there are a number of shows where I prefer the overall dub to the Japanese not for one particular voice or the other, but for the entire ensemble. That being said, Cowboy Bebop, Trigun, Wolf's Rain, and Ghost in the Shell all fall under that umbrella, but I'm also a big dub supporter in general, and there are plenty of other dubs I find quite excellent, too. (Evangelion, FMA, Death Note, Code Geass, etc.)