There are so many wonderful memories associated with the game that it's hard to pick a favorite. But if I'm forced:
My first experience with the game back in open beta. I saw a link to the beta sign up and my thought was "Well, this looks silly. Still, it won't cost me anything to try it. I've got some time to kill..."
Within my fist hour, I had decided I'd found my new home. It hooked me in that fast.
While this memory in itself doesn't have much emotional weight of its own, it is the one without which all of my other great times with the game wouldn't have been possible. A simple whim on something that didn't really interest me, but I had some time to waste. God, I'm glad I "wasted" that time.