But of course I'll enter!
My Seeker and Loremaster titles were the most important thing to me. I loved going through all the lore, instead of speed racing to level so I could raid. Seeing everything really made me appreciate the game on a whole new level.
I think the best was my first tier piece back in Wrath. The warlock head piece in T10 was so very hawt! And the agonizing speed at which I acquired the Frost badges. So so so worth it!
I love the new loom changes :p
"Insane in the Membrane", before the change. :)
I will never get Eye of Sulfuras :/
Benediction with the Lvl60 priest quest. This was epic !
My Unpainted Mechanostrider, I take it for a spin once in a while, what a beaut.
Finally getting 10-man Firefighter down with my guild after working on it for weeks was quite a rush. Also, getting my Dragonwrath from Firelands was pretty memorable.
My favorite moment happened when I found and killed Aeonaxx! Lots of jealous people that day!
Mini Tyrael is just so sexy. Also, he would be the perfect companion to my Mini Diablo from the Vanilla CE!
I think the most I was excited was when I got my first drake mount. Who doesn't want to ride a dragon?
the "Glouries!" achivement was a very great moment for me, because i spent alot of time on farming every rare in MoP
To get my very first (weapon) Legendary Item!
I would have to say...What a long, strange trip it's been is the most exciting achieivement I got. When I got it last July I was so happy after doing all the holiday's I finally got it. I was elated!
Fiery warhorse reins, still farming, still no luck.
For the longest time achievements have been my sole purpose for playing the game but the one thing that made me burst into random screams of joy was finally getting my precious spectral tiger :P
The day I received my first glass of yeti's milk.
most recently, getting the Ashes of Al'ar. Or maybe when i got my White Polar Bear, which was the first mount i actually wanted to get enough to spend time farming for it ^^
I Can't Hear You Over the Sound of How Awesome I Am, Heroic: Ragnaros and Reins of the Raven Lord are all pretty high up there for me.