My best WoW memory. That is fairly difficult to nail down. For the sake of entiretly I think I'll talk about some of my favorite WoW memories and maybe in that process I'll figure it out for myself.
One of the earliest awesome memories I had was hitting the level 70 cap. I would have to say that was my real first accomplishment (or feeling of accomplishment) I experienced in game. The grind to 70 was very and very hard, especially for someone who had yet to develop a desire to run instances. I think all throughout my first 70 levels I did maybe 2 instances, one being WC (and still was a wipefest) the other being Underbog. Getting to 70 via questing alone was quite a feat back then, the experience cap had still not been lifted at that point. As I level characters now using BoA epics with XP boosting qualities and a lower overall experience cap, I remember less than fondly the feeling of wanting to claw my eyes out from level 66-70. There were times when I seriously said "fuck this game" and just took a few days off - leveling was just that painful. I contribute some of that to not really knowing how to play that well (I would only do green with the occasional yellow quest) and to questing mainly alone. You can bet your ass when I dinged 70 I did a little dance.
Another awesome WoW memory I have would be killing the Lich King for the very first time. I'm sure you are thinking, why is something that common a fond memory? When I first killed the Lich King that 10 man group was the very first group to kill him in our guild - to me that means something. Back then my guild was running two 25 mans and four (sometimes six) 10 mans a week. To be the first 10 people with that title and that achievement felt good. At the end of that fight when LK kills us all, the roar that came from vent and myself was deafening. This isn't to say that subsequent first kills are not as satisfying, but something about being the first 10 people to kill the expansion end boss just puts a smile on my face.