Subbed >>>>>>>>>>> Dubbed. Sure half the Z fighters dub the funimation dub, but they don't have the pure emotion backing up the story. Also. Luffy's dub actor is annoying to me. Hes the main character. You are suppose to enjoy listening to him talk.
I do say that the recent dub episodes that were released(aka the Water 7 arc) were pretty decent. I wanted to see how the voice actor of Usopp/Krillin handled a certain part. He did it pretty well. I could see some mistranslations but he had most of the emotion in that scene down. It wasn't half bad. The earlier episodes they dubbed aren't as good though.
Whatever you do though, DO NOT watch the 4kids dub. I don't care how much you like the rap opening, don't do it.
Also, there are videos on Youtube that compare all 3 versions of one scene. It might be spoilers but if you really want to know for yourself which is best, look at those.
[video=youtube;oyNwfcvf4bo]http://www.youtube.com/watch?v=oyNwfcvf4bo[/video]
Here is a video of the 2 dubs. Its a filler episode so it doesn't spoil much for you. I think this is the right channel for the sub comparisons too.