I think mine would be Loremaster. I always wanted to get it and when I knew that there would be new quests with Cataclysm I decided to do it, I hated BEM and doing Kalimdor as Alliance, but I got it done!
Dragonslayer or Immortal. Dragonslayer for being a badass sounding title (and fitting for most expansions!), and also because we got the Sinestra kill as server first. Immortal because it was a hard title to get, and only for limited time.
I don't really see many as all that prestigious, although I haven't seen anyone with The Immortal in a while now.
Honestly I think the best titles are the ones that fit well with the character names. Some of the people that make their names to fit with certain titles make me giggle.