Here's my favourites, despite their cons I'd still rate them a solid 10 what they achieved at the time of their release is irreplaceable for me although I accept that now they've dated and not always in a good way, personally I'd still play them now, over and above many new games which have tried to emulate their game mechanics.
Outcast -
Pros: Immersive and expansive world, characters, narrative, great score, fun combat.
Cons: No hardware acceleration for voxel graphics meaning if your CPU wasn't up to the task then the game was slow.
Thief -
Pros: Good storytelling, dark quiet levels where every sound is a clue as to what's around the corner which increases suspense and tension. Great main character and well thought out fantasy world.
Cons: Undead levels were meant to be shooter-esque and failed miserably.
System Shock 2 -
Pros: Scary, well-thought out continuously changing environment, characters and narrative with fleshed out side stories and a long play time.
Cons: Combat is tough and the fatigue on weapons is annoying after a while.
Deus Ex -
Pros: Diverse, wide-ranging levels, in-depth NPC interaction, involved and intriguing narrative, choices made in-game effect the rest of the game.
Cons: Combat was on the verge of broken once you ranked up and animations can look too robotic sometimes.
Half-Life -
Pros: Detailed immersive environment where situations hint at a larger story yet to be told allowing your imagination to run riot. Diverse and frenetic combat. Many levels, situations, overall quite a long game if you explore everything.
Cons: Lacked direction toward the end of the game.