My preferences run in this order:
1) A well done dub - ie. skilled, professional voice actors that fit the character
2) A well done sub - ie. subtitled text is an accurate translation and has correct spelling and grammar
3) A badly done dub - ie. Poor audio quality or mediocre voice actors
4) A badly done sub. - Spelling and/or grammar seem to have been done by someone barely familiar with the language
I'll highlight a failure that pushed one anime from a 1) to a 3) in my mind - the Negima!? anime, For some reason, they made Negi sound like a snooty young English professor. This was annoying by itself, but especially irksome since his character he grew up in Wales, not England. To make things worse, Evangeline sounded like a haughty English noblewoman. She was't English either though. They never indicate where she was from exactly but eastern europe would be more likely.
If I have one minor peeve it's that some type 1) anime still has voice acting where you can't quite make out what someone is saying on occasion. Switching on the subtitles doesn't help since they are a translation of the japanese dialog and not the localised dialog that the actor is speaking.