Thats actually a tough one for me.
It depends on the animation really. I HATED the english version of Naruto with a passion, I cant even watch it. It was great subbed. Some anime have such horrible Subtitles its just sick. Like DNangel and DNangel are good examples.
I liked Tenchi, Gundam Wing, Gundam The 08th MS Team, Trigun, Full Metal Panic Dubbed (theres a lot more but I cant think of anymore at the moment). I usually watch anime Dubbed(unless It's too awful to watch) then Subbed later on. Some are better Dubbed and some are better Subbed.