I don't have a preference. I'll usually make my decision on a case by case basis. Like, Dragon Ball? I always preferred some variation of the US dub, mainly because it's what I grew up with. Same for Afro Samurai, Cowboy BeBop, Lupin III and Fullmetal Alchemist.
Samurai Champloo, on the other hand, I usually watch in original Japanese with subtitles, mainly because I have a hard time taking Steve J. Blum seriously in any role other than Spike Spiegel. Also, the US dubs of Tenchi Muyo, Sailor Moon, Yu Yu Hakusho and most of the Gundam franchise (except maybe Wing) are pretty atrocious.