Final fantasy 7. While it wasn't my favorite game (is near the top) It was the best experience. Though to be honest it was more the entire situation in my life.
...unless you can count World of Warcraft. The first 2 years of playing that, the community, everything involved...
I guess really the games are only part of why they have such a great memory...
but I can't think of a better one (other then wow) then opening up my ps1, ff7, on christmas, and spending the next 3 days around family, great food, and playing non stop.
However if we are talking about just the game, none of the multiplayer, community or feelings around the game at the time...just the game its self...
would have to go with...mechwarrior 2 or total annihilation.