Devil May Cry 3: Special Edition, it was is and still my all time favorite game, thank God for the recent HD Collection, which i already platted some time ago.
Zone of the Enders: The 2nd Runner
Metal Gear Solid series
Hitman series
Ninja Gaiden remake series
The Witcher series
Marvel vs Capcom series
F-Zero GX
Picking just one is impossible, all of these games have their different charms and reasons why they are my top games of all time.
For now, God Hand is my number #1. Boss soundtrack, boss characters, boss gameplay, boss difficulty, boss... boss fights? Hell, EVERYTHING is manly in this.
.....jeez it smells like mothballs in here. Mine probably has to be Fallout 3, the sheer joy I got from the first 3 playthroughs has yet to be matched on such a level.
Super Smash Bros Melee, Super Mario 64, Legend of Zelda: Ocarina of Time, Starcraft, and Final Fantasy 7 are my favorite games ever. I've pulled all-nighters with these gems.