Very hard to give only 5 games, depends mainly on the kind of game? here is a list, not really ordered
Stalker: Call of Pripyat: It took several opus to create the "final" Stalker but it seems they achieved a near ideal game. a bit short though.
Jagged Alliance 2 v1.13: fan improved version of an already wonderful game.
X3: Terran Conflict: very interesting genre, too bad there aren't more games like that, very good community making wonderful mods.
Zelda: A link to the Past, my personal favorite zelda game
Fallout 2: Very good atmosphere, real RPG and post apocalyptic setup. Yummy
Honorable mentions (more or less ordered by "honorability"):
The other Stalker games, Shadow of Chernobyl and Clear Sky, wonderful games too but far too bugged to be in the top
Deus Ex, maybe the best RPG/FPS or at least the most RPG one.
Team Fortress 2, one of the best multiplayer fps ever made
Other Zelda games (especially Ocarina of Time)
Warhammer 40,000: Dawn of War II, very good real time strategy game, unlike DoW it is quite close to Warhammer 40k because of its squad based combat.
Heroes of Might and Magic serie, especially 3 and 5, very good turn based strategy games.
Baldur's Gate serie, wonderful RPGs but a bit overcomplicated, especially for Mages.
Fallout 1 (I actually played the 2 a lot more, which explains it is not in the top even if it is usually seen as better than the second opus)
Crysis very beautiful and fun to play (must be played in the hardest difficulty level though.?)
Marathon serie, Myth 2 and 1
Escape Velocity Nova
Ok, I think I'll stop here or it won't end