Starting to down heroic Cataclysm content without being Cataclysmic-ally Epic. 12 for 12 on normal, with Halfus down, and I still don't have some shoulders to complete the achievement (OR my tier bonus!)
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I'd say my favorite was Realm First! Cooking Grand Master back when WotLK came out. It was a several day battle to get the needed spices, especially after losing out on Realm First Fisherman.Though my Accomplished Angler is right behind it.
My Favourite was hitting 100 mounts and getting my blue dragonhawk Blue Dragonhawk, up to 110 now :Pstill no aeonaxx tho.....
Vial of the Sands is one of the most expensive achievements I've earned. Flying friends around while questing makes it worth every copper!
I liked Lich King, so Bane of the Fallen King it was sweet.
I was happiest about my For the Horde achievement, which was the first raid I had ever organized by myself and led to the successful sack of four Alliance cities and some cute little bears to ride around on!
I am proud of my World Explorer achievement that I got on my priest at level 69 with a ground mount. Icecrown and Storm Peaks was horrible. :o
the achievement i'm most proud of is a 3 way tie between my epic Warlock, Paladin, and Druid mount quests. doing those back in the day was a challenge and very expensive, but i'd do them all over again if i had to.
The one achievement I was most proud of was The Twilight Zone (10 man), because I got it while it was still a very difficult fight, and the icing on the cake was rolling 100 on the drake! It felt awesome to have a hard-to-get mount and achievement when people spent months trying and failing it.
Bloodsail Admiral. :D
While it doesn't give any points, I'd have to say my WoW 6th Anniversary Feat of Strength is my favorite. I've been at this game a long time, and seeing it reminds me of all the good times, and my the friends I've met along the way.
I'm most proud of my 100 mount achievement. I worked my tail off to get it and it took forever. I needed not only a whole pile of rep, but also tons of money. And I did it all before the argent tourney which made it even more special for me!
For me the most proud I've ever been at receiving an achievement was the other night when I finally got To Hellfire and Back. It seems like it should have been really simple, but for the longest time I was stuck at 71/76 quests completed. At the time I could not, for the life of me, find anymore quests to even attempt to do. Then the other day I figured I'd head to Honor Hold since I was now level 77 and see if anything else had popped up. It had and I received exactly five more quests, which bumped me up to where I needed to be to get the achievement. When I hit the 76th quest mark I felt so relieved that I didn't have to see that 71/76 anymore. It's definitely been the most elusive one for me so far, so it tops the list. I am also ridiculously happy that I am DONE with that area. Hellfire Peninsula, you are the most frustrating area I have yet to encounter.