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.
use to put scarab lord with these until a bunch of people xferred for it.
old server first titles "can" be prestigious or you "could" have gotten it from a dead server in which case it wouldn't mean as much
but anyway I think immortal sounds the coolest also thought I was a bit disappointed how long they let people go back to get the achieve in tiers later (yes I know others feel differently and that's cool)