The sentimental value is kind of loosely defined here but for the sake of this thread these are games that were what I was playing during important milestones. Or just really good and memorable games. And I know I'm forgetting a lot of them from my youth. I had quite a collection of NES and SNES games but lost most of them through the years and it's a pain remembering them all. Some of the teenage years are fuzzy too, especially Playstation games.
Youth:
Battletoads (NES)
Super Mario Bros. 1-3 (NES)
Metal Gear (NES)
Paper Boy (NES)
Legend of Zelda, Link, etc. (NES)
Pitfall (NES)
Jurassic Park (SNES)
Doom (PC)
Teenage Mutant Ninja Turtles (NES)
Tetris (PC)
Ninja Gaiden (NES)
Metroid (NES)
Duck Hunt (NES)
Earthworm Jim (SNES)
Ironsword: Wizards & Warriors II (NES)
Final Fantasy Mystic Quest (SNES)
Final Fantasy (various titles) (NES, SNES)
Sonic the Hedgehog (Sega)
Batman (Sega)
Teens:
Mortal Kombat II, UMK3, Trilogy (Arcade, PSO)
Tomb Raider 1-3 (PSO)
Spider-Man Venom Maximum Carnage (SNES)
Total Carnage (SNES)
Resident Evil 2 (PSO)
Turok (N64)
Final Fantasy (various titles) (PSO)
Redneck Rampage (PC)
Killer Instinct (Arcade)
Adult:
Alien vs Predator: Gold, AVP2 (PC)
Star Wars Jedi Knight: Dark Forces II & The Mysteries of the Sith (PC)
Star Wars: Knights of the Old Republic (PC)
Grand Theft Auto III, Vice City, San Andreas (PS2)
The Darkness (360)
Splinter Cell series (X-Box)
Halo 1-3 (X-Box, 360)
Elder Scrolls IV: Oblivion (360)
Gears of War (360)
God of War (PS2)
Doom 3 (PC, 360)
Legend of Zelda: Twilight Princess, Wind Waker (GC)
Kingdom Hearts (PS2)
The Thing (PS2)
Blood Rayne (PS2)
World of Warcraft (PC)