My favorite is definitely The Immortal. Our guild managed to achieve this when the content was still being run regularly. By the time the achievement was taken away it was only our guild on the server that managed to achieve it. While that guild has since been dissolved and many of my favorite raiders have spread across the WoWverse to different servers, I still wear that title and think fondly of those times.
This post was from a user who has deleted their account.
I liked The Frozen Throne (25 player) due to the how epic the fight was in terms of lore, considering Arthas is/was one of the biggest lore figures in Warcraft history.
My favorite achievement is the Winterspring Frostsaber. What a painful grind!
Thunderfury, Blessed Blade of the Windseeker! Having to farm Molten Core to the edge of madness, week after week... and then.. the day you least expect it... the left side finally drops. Immediately teleport to Dalaran, take portal to Tanaris, Flight Path to Silithus and THEN fight http://www.wowhead.com/npc=14435#screenshots:id=84324. When you finally end the quest, equip the mighty sword and then the people next to you asks, again," Why on a mage?" :D I love that achievement + weapon.
No doubt! To All The Squirrels I've Loved Before
My favorite is also What a Long, Strange Trip It's Been. It is also my least favorite as it took me several years to do (missed a few holidays). But yay for "free" 310% flying.
The Loremaster, because it means you have seen just about everything. All the interactions between NPCs,and the new ways that zones change as you quest through them. Having played since Vanilla it's nice to see the changes that have occurred everywhere and to actually feel like your questing in a zone actually helped with your factions advancement.
At the risk of being criticized by the "lifers", I'll be honest here. I think I'm actually most proud of That Takes Class...specifically because of how initially fearful and apprehensive I was to ever engage in any PvP play.
Proud of my Ashes of Al'ar; mainly because of what I had to go through to get it. After hitting 80 in WotLK, around 7 people and myself went to farm the mount, after just two weeks it dropped and I rolled a 96 on it, but a guy had switched to master loot at the last second and took it from me. I wasn't too mad about it, but the next day he went through several GM tickets before he finally found a GM who would transfer the mount to me after he took it.
The Seeker title, for completing 3000 quests, on my mage. Yes, it isn't the most glorious achievement, but it certainly seemed to take forever. Having started playing WOW right after BC launched, I slowly toiled my way to 70, grinding out quest after quest. When Cata dropped, I was dismayed to find that quite a few of the quests I had done were simply... gone. I can't remember how many I lost credit for, but it was disheartening. I put my head down and kept bulling forward though, and managed to pick up other achievements along the way. I proudly display my title, a title that I alone earned with the help of no other.
Without a doubt it'd be Did Somebody Order a Knuckle Sandwich? because I got it on my warlock...because I COULD.