What achievement are you most proud of?

[Augusta138]

I've got a few on my list.

Amani War Bear: one of the best group experiences i had in early WoW
Ahead of the Curve Garrosh: The experiences about playing MoP and with the people ive met during this expansion are worth a lot to me.
Hunter MC class weapons: They were THE hunter artifact way before artifacts. loved doing it.
Both classic Zul'Gurub mounts: still use them a lot today, still proud.

[Coolthulhu]

Salty when winning the STV was required, solo, on a PvP server with groups of dedicated gankers helping other competitors. I'm fairly sure I wouldn't have made it without alchemy (invis and living action potions) and engineering (nitros). One guy's gank group especially must've regretted not having their warlock cast detect invisibility near the turn in, hah.
Breaker of the Black Harvest was also rather satisfying though nowhere near as challenging as some other stuff including the STV extravaganza.

[chrisnumbers]

The Immortal.
We got it in the final week before its removal.
phew!

[Glowbear]

Camel-Hoarder. Took me literally months and over 35 fake statues before I got the real one. It brought out some seriously obsessive behaviour in me - staying up late to check low-pop realms, making a map to track where and when I'd seen spawns - but it felt great to get. I use the title on my alts but not the mount.

AOTC: Garrosh, which I got by persistent pugging, having only started playing WoW in 5.4.

AOTC: Xavius, my first and to date only curve achievement from raiding with a guild.

[Deleted]

Hands down(all the other achievements ive done were less trouble for me):

Show Me Your Moves in MSV(requires 10 people not to fail something pretty damn hard).

Exalted with Hydraxian Waterlords(takes about 14-20 weeks weekly farm, depending on how hard you cheese it).

Exalted with Orgri'la. Just stupid and boring.

Doing Argent Tournament and getting every mount possible.