If it has to be one:
Wow - from original release day up until TBC release. The levelling, guilds, dungeons, pvp, gearing, getting your first epic mount, the first time in molten core with 39 others, seeing a thunderfury for the first time, even the general chat... the list goes on and on.
The most mind blown was probably playing the original Resident Evil on PC though, which I believe was one of the earlier games to require a gfx card to play. Seeing that first zombie scene in the mansion shortly after the intro video and you just knew it was going to be like nothing else.