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!
Gladiator, despite all flavor of the months, broken items and and general overpoweredness it still takes a lot of insight and dedication to get.
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.
The only titles off the top of my head that catch my attention are the insane and scarab lord. I don't really notice much though.
ambassador (with silk cloth (in begin of wotlk)) and Zookeeper <3
I'd love to have Scarab Lord, Death's Demise and the Celestial Defender.
If I just started playing earlier....
For now Insane will do it for me.
Mine is Death's Demise by far.
Loremaster (pre-Cata) on a DK.
My FoS Stone Guard title is most dear to me. Though I'm bitter I never got the Blood Guard title back then (I have the new one, but it's not the same).
Dragonslayer, My God I love that title so much!
My druid enjoys Guardian of Cenarius, quite fitting and was fun to get :] Bonus points for Raven Lord mount (it was a pain in the ass to get!).
8/9 realm best CM times on US-Mal'ganis on my mage. And my old Gladiator title on my warrior.
I dont attach prestige to titles.
I like Starcaller though. And The Love Fool.
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.