This question took me a while. I had to think about it and really look through my bags and bank space. But then I realized that my favorite item in the game is probably actually two items.
My moonkin hatchling and my Teldrassil spoutling. Or as I call them, my mini-me's.
I'm not a big collector person but as soon as the Argent Tournament pets came out, I begged my husband to trade for a mini-tree druid. He managed to get his hands on one for me and I raided through ICC with it. I was really kinda weird about it. It was always out, I loved the way it would snore as I was running around casting like crazy.
My moonkin hatchling was a Christmas present from my husband. I had been dropping hints about it but I never checked my email to see if there was a code in there for me. So all Christmas day, I'm whining about how much I want the pet and lo in my email it sat. He eventually asked me pointedly "Did you check your email today?" and I ran off to do that. My moonkin is out almost all the time now. I prefer it over my mini-tree druid as until recently I didn't have the glyph of the treant. Plus, it flys along side me, which is just about the most adorable thing ever.
13 hours ago