Ion Storm was a company as well-known for the names on its payroll as for its comically extravagant office space and outlandish development costs. But despite some questionable management decisions, they managed to turn out several high profile titles in their short existence. The one that easily stole the show was Deus Ex - a dystopian science fiction tale set in a world where every conspiracy theory imaginable is real. It was also an early example of a game where one's choices actually matter, with significantly different play styles depending upon the player's choice of skills, story events changing based on choices and even three different endings. A compelling dark tale in a surprisingly credible future, there's a good reason that it's the centerpiece of a meme: "Every time someone mentions Deus Ex, someone reinstalls Deus Ex. "
9. Half Life 2 (Valve Software, 2004)
8. Duke Nukem 3D (3D Realms, 1996)
7. Ultima V: Warriors of Destiny (Origin Systems, 1988)
There also exists a rare, network-enabled online edition of the game that allowed up to four players to act as 'city commissioners', operating on the same map and exchanging resources with one another to work toward their own goals.
5. Planescape: Torment (Black Isle Studios, 1999)
4. Thief 2: The Metal Age (Looking Glass Studios, 2000)
3. Ultima Underworld: The Stygian Abyss (Blue Sky Productions, 1992)
A game that was built from the ground up to not just be a more realistic kind of puzzle-driven dungeon crawler, but a full blown life simulation too. To that end, you had skills not just oriented around combat and spellcasting, but for swimming, conversing, identifying items and bartering with NPCs among many others. It had a lighting system and rudimentary physics for platforming, letting objects bounce off of walls (and activate switches) and no single set solution for most puzzles, letting the player take an innovative approach to figuring out the game's mysteries. Downright mind-blowing stuff for 1992, and the influence it's had on the industry since is immeasurable, inspiring games like Elder Scrolls, Half-Life 2, Deus Ex, and numerous others. It's a bit clumsy and awkwardly slow to play today, but it's nevertheless a great game and an important building block for gaming as a whole.
2. System Shock 2 (Looking Glass Studios/Irrational Games, 1999)
1. Ultima VII: The Black Gate (Origin Systems, 1992)