Definitely when I got my first 100 mounts. I'm now obsessed with getting every mount possible, so every time a new one comes out I'm excited all over again!...and achieving whatever is required to get said mount is always fulfilling. It seems my quest to have them all is never-ending, but that's part of the fun :-)
My white chocobo!
Hands down it is the "Is Anouther Man's Treasure" achimvent for the title: The Relic Hunter. Getting to explore places in those tiny cracks to find the BoA items, was very enjoyable.
That would have to be Rivendare's mount. I had been farming Stratholm since Vanilla and I didn't acquire it till Wrath.
I was so happy to get my turbocharged flying machine. These damn khorium power cores were the worst because I was low on cash and khorium is so rare. And the adamantite frames. I actually waited for the vendor in area 51 to restock for them. It is still one of my favorite mounts. Engineer is the best.
I got Amber, Ruby and Emerald Void by soloing Eregos after dismounting my Drake.
feldrake is nice:)
My most recent acquired item is the Cinder Kitten. I was glad that I could do something...
Getting Realm first Illustrious Alchemist and Realm first Zen alchemist. It's such a rush to see that achievement pop up, followed instantly by tons of congratulations and also followed by a swarm of very angry people who were probably only seconds behind me. Takes a lot of persistence and a LOT of gold, and every skill point closer to the final one gets more stressful, just hoping someone isn't beating you by just one single potion or transmute. For the Illustrious I had to acquire the help of many friends and guild mates who were willing to not focus on leveling so they could mine ore and pick herbs for me. Of course, for holding them back from leveling on their brand new expansion I payed them generously. For the Zen Master one...I was pretty sure i was going to have it, but before the server's went live for me I got called into work. I was pretty upset about it and couldn't keep my mind off of it, however when I got home I saw that my wife had jumped onto my account while I was out and nabbed the Realm First achievement for me. Many thanks to her for enabling two Realm Firsts in a row. I, of course, had to repay her generously as well...So i bought her the Ethereal Soul-Trader pet. The first one to be sold on our server once they were allowed to be boxed up due to the new systems. I think it was a fair trade, because it was the pet she wanted more than anything on the game, but we were always deterred by the 200$ price tag on it. Luckily I had enough gold to grab that one (well, once i borrowed some more from someone). The purchase pretty much emptied my pockets of every last copper, but the realm first achievement made it worth it.
Hmmm i would have to say my 2008 spirit of competition vanity pet and tabard.
Probably raiding the relevant content in Wrath of the Lich King (Naxx, Ulduar, ICC) with friends of mine, as well as getting to experience the heroic modes. One of the only times I felt intense amounts of anxiety while healing was during Heroic Sindragosa.
Being just over a year old to wow, it was gaining "What a long, strange trip it's been". Made a true friend shortly after joining and every world event we would work on getting every achievement necessary and some extras for fun. The true excitement is both of us getting the achievement at the same time. A year of fun followed with a new best friend!
The title Zookeeper, but I still have to wait until the summer before I can get the last pet I need.
Getting the unstoppable force after fighting in all night long AV's was pretty satisfying.
It's hard to pick what achievement was the most exciting. But my favorite to get was Higher Learning and getting the book that teleports you to the tower. I'd invite one of my friends and port there then summon him there. It was funny :)
Main-hand Warglaive. Got that other one to drop 3 times in the times it's taken to see the main hand one
Just give me all the prizes ! xDD
Soloing my first Cataclysm dungeon - after getting a kicking or two from them when Cataclysm first came out it was fun going back in and getting a little payback on those bosses!
My favorite acquisition has got to be my Swift Zulian Tiger. I farmed that place killing only that boss for roughly 3 and a half years, hardly ever missing a lockout, and YUS! it was a PITA at lvl 70 to kill him alone (even as an OP druid tank). I finally got this mount (iirc) about a week, maybe 2 weeks before old ZG was removed. such an awesome day.
Obtaining the incredibly good looking challenge mode gear on my paladin, and getting flametalon of alysrazor!