Silent Hunter Series especially from 3 - cause coming from SH1 and 2 then seeing the graphics of 3 just blew me away at the time and i found myself playing SH3 for 2 years solid, still play it today. Now with SH4 the graphics are just pieces of artwork!
Planescape Torment - RPG. Old but had an adult theme, you played a character that was dead. The conversation dialog between characters were the best ever.
World of Warcraft - mmorpg cause its addictive as hell, makes you worry, makes you get up hrs before sunlight to level up that bit more before the day starts, you start buying books on Wow, reading about new ways to beat the game, learn a new history and can even teach yourself a new language - (bloodelf) epics and the right gear your character needs for raids, you learn skills like engineering and you appreciate daggers much much more than you would of.
Deus Ex. RPG. It gave you choices that seem to effect your way of thinking in the real world - hmm should i have done that - guilt trip starts to settle in:rotfl:
System Shock 2. For freaking me out
Doom 3 
I know, but all the same it scared me :rotfl:
Su-27 Flanker - crappy graphics now but the first realistic flightsim to come out. Beats the stock Falcon 4 by miles.