20. 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.
15. Thief Gold (Looking Glass Studios, 1998)
14. Baldur's Gate II: Enhanced Edition (BioWare/Beamdog Software, 2013)
13. Ultima IV: Quest of the Avatar (Origin Systems, 1985)
12. Undertale (Toby Fox, 2015)
11. Fallout 2 (Black Isle Studios, 1998)