The first time I watched Cowboy Bebop, back in the early 2000s, it would have only been the dubbed version available on TV - so I guess I have some nostalgia for the dubbed version of the show. The same for Outlaw Star. With that said both are regularly referenced as excellent english dubs, so I don't think it's just nostalgia goggles. I have watched the series in Japanese with subs as well, and it's fantastic either way. I don't think I'd watch an english dub of a show I hadn't seen when I was younger, though - I'd just watch the Japanese version with subs and not bother with the dub. I've read that Death Note, for example, has a really good english dub... but I watched it first in Japanese and therefore have no real reason to watch the dubbed version as the Japanese voices are pretty much perfect anyway.
Weirdly a lot of lists have the Dragonball Z english dub as one of the best. This either means there are very few good english dubs of anime out there, or a lot of people are clouded by nostalgia, as DBZ english dub is pretty shit until quite late in the series. It had improved a lot by the end, but 70% of it sucks, particularly during the Saiyan and Freeza sagas.
I've heard Samurai Champloo is very good - written by the same guy who did Bebop, I think? I should watch that too.