Generally it's a rule for me that if I haven't seen a subbed version of something, I can tolerate the dub just fine. I used to really like Tenchi Muyo's dub, but after hearing the original voices I just... blegh.
But there are a handful of anime that I prefer dubbed regardless, usually cause they don't have any lolis. Young girl characters always have the same terrible sounding voices in anime dubs.
But at least they don't do what the biggest contender for my dub preference did. Dragonball Z's Japanese version had characters such as Goku, Gohan and Krillin keep the exact same voices they had when they were little kids, so you have this grown man with bulging muscles shrieking like a five year old with a cactus stuck in his throat.
At least the dub was kind enough to give them adult voices.
Another reason I prefer DBZ dubbed is because of nostalgia factor. The music in particular is so ingrained in my memory that even the Kai dub didn't feel the same.
The first season of Digimon is another I prefer dubbed, mainly due to nostalgic reasons. Particularly the voice cast. Same goes for the early Pokemon seasons, though once they recast the characters all bets were off.
And now for the ones that I will only tolerate subbed.
First we'll get the obligatory 4Kids mention out of the way, with Pokemon perhaps being the only exception. Ojamajo Doremi definitely belongs subbed and Sonic X for the love of god is a much more rewarding experience subbed. The soundtrack to the original was one of the best things about it, but the dub turned it into generic renditions of like... one or two themes. The dub voice cast didn't help.
Shinryaku Ika Musume is probably the biggest non 4Kids example that I adamantly refuse to watch dubbed. The title character speaks with a verbal tic like a Precure mascot critter. She ends her sentences with "-de geso" which has something to do with squids.
The dub, (and some fansubs) can't really translate this into English, so rather than leave it out like decent human beings, they decided to have her speak in the most painfully unfunny squid/ocean based puns imaginable.
"Where do ya ink you're taking me?"
"I'm not going to krill you!"
"You gotta be squidding me!"
The fact that the official DVD release uses the dub's script doesn't help.
While the first season of Digimon was tolerable with its dub, the second season was terribad. They turned it into an outright gag dub, making it impossible to take the plot seriously with the increase in cheesy one-liners and destruction of any silence that lasted more than a second. Probably the only bright spot in it was Steve Blum's portrayal of Blackwargreymon.