Personal favorite: toss-up between Planescape: Torment ("What can change the nature of a man?" is to this day pretty hard to beat as a main theme; great writng and characters, all rounded up by AD&D 2e) and Fallout 2 (probably closest, though still light years away, that video game RPGs got to pen&paper RPGs; good story, memorable characters, fantastic setting and great GURPS-ish alike RPG system).
Most influential: most likely Doom
Most ambitious: probably Ultima Online or TES: Daggerfall
EDIT: The "why" part addedLast edited by HoloDust - on 27 January 2020