Rome Total War (shout out to Medieval II),
Pokemon Red/Blue, although I thought Fire Red was a very good remake.
Star Wars Battlefront 2, purely for the online play alone.
The early days of Runescape before Jagex ruined it.
And althought it's not cool to think that CoD is good, World at War.
Diablo, without a doubt. An incredible game, and a huge milestone for it's time that has defined so many other games. Can't wait for Diablo 3!
Some other games to get a worthy mention : World of Warcraft - Mainly vanilla, due to it's shear innovation - that game up to WOTLK consumed so many hours of my life. League of Legends - Might not seem an obvious choice, but for a free to play game it's incredible, and great fun too! Runescape - Early runescape was great, no shame there!
I have no compunctions whatsoever about saying that. A few months ago I might have said Half-life 2, Portal, Halo or EVE.
But whilst I have a great love for many games, one now stands out above all others. Journey is just sublimely beautiful, it is three hours of gaming and emotional perfection, with the best score to ever grace a game.
And I will probably not see it bettered in my life time.