If I had to choose just one... oh that would be really painful. Still, it would have to be Altoholic. I have so many things spread across so many characters and gbanks that I would weep if I had to keep that information all in my brain.
Close second is a tie between Classic Quest Log (returns the look and functions of the quest log to my sanity) and Z-Perl (because Blizzard can barely do UI out of a wet paper bag).
Honorary gold star to BadBoy: Spam Blocker & Reporter. It's the CrapCleaner of Trade channel. ❤