Although it's not an intro per se, I'd have to say the most impressed I've ever been with a game was when I first played World of Warcraft (at the first public stress test) when I walked into the gates of Stormwind for the first time. After playing various MMOs where the "cities" consisted of a few scattered buildings sitting on top of generic landscape, to see something of that size and level of detail was incredible. Say what you will about WoW (I actually haven't played it in years now) but you can't deny that Blizzard sure knows how to make an environment.