Fallout for me, i remember watching my dad play it..he wouldnt let me since i was too 'young' at the time, but soon enough i got my hands on it. It had just come out at the time and i was amazed at the graphics.
Then i got Fallout 2, noticed it in a shop for around a tenner and i thought i have to get it since Fallout 1 was so epic.
The whole idea of it owns to be honest, probably why its arguably the best RPG ever.