No titles are really impressive anymore, almost all of the 'prestigious' ones can and have been cheated in some way. Gladiator and rank 1 titles have been sold since the beginning, MMR exploits, win trading. More than half the Scarab Lords in the game were people who got them in WotLK and plenty of realm first kill titles were achieved months after the bosses were actually relevant.
My favorites are in order: Herald of the Titans (done in Wrath), Salty (took me FOREVER to catch a rare), Chef (first one I got that took a lot of effort, done as soon as it was possible), Insane (post shendralar).
Most prestigious title overall: Herald of the Titans
This title was cheesed before Cataclysm with the stat inflation patch, and there was a bug where if you killed him fast enough at lvl 90 in any gear you got the achievement. I find it rather cheap now tbh.
Zookeeper is cheesy sounding, but it was the first thing I really went for in the expansion (up to guardling, of course) and I wear it on the character that was the pet collector before we even had Pet Battles. I think it is only prestigious to those of us that pet battle.
That being said, I also like Battlemaster and Khan, since it is a grind with achievements twinded in.
My roommate just got Hand of A'dal a couple months ago... while smashing those places as a geared 90 lol so meh on most of those "just go get it" titles.