I've only been playing a couple of months, but getting my raptor mount from They Grow Up So Fast after 20 straight days was pretty cool.
Agent of the Shen'dralar, I suppose.I worked hard to get the Insane but because I started so late I ended up getting Insane in the Membrane just after they removed Shen'dralar as a requirement.So I'm happy to have that reminder and recognition. ^^
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For me it is definitely http://www.wowhead.com/achievement=5108. Just because it grants the obnoxiously long tittle "Slayer of stupid, incompetent and disappointing minions".
For me, it would have to be Glory of the Ulduar Raider (10 player), as I was able to complete it while the content was still relevant, not an easy task at the time, as well as I was the raid leader for the runs. Truly feels good to have a permanent reminder, the drake, of what I and my team were able to accomplish.
I just love my Shave and haircut Achievement!
I'd have to say Accomplished Angler. But more specifically Master Angler of AzerothWinning the fishing contest in stranglethorn was quite a rush, and I think I'm going to wear my salty title forever.
My The Exalted title
My favorite achievement is For The Horde. I like it because of the social experience it was of managing 40 people to assault all the different cities.
As a Paladin, I'm most proud of my Charger achievement (Feat of Strength). The quest line was mildly painful: collecting materials, gathering gold, and finding a group to run Stratholme. However, in the end, it was worth it for one the best looking mounts in the game!