This is year I finally got my own computer (AMD Athlon 850 + Nvidia TNT2 ultra) that I did not share with my parents and along came Diablo 2 to fill my nights. Diablo 2 was probably the first PC game that I play none stop for months so it get my vote. It also the standard that basically all games after it followed in this genre.
My runner up is Dragon Quest 7. I pretty sure this game have easily my record for longest first play through of any JRPG I played. Other JRPG I have spent more hours in when counting multiple play through or post end fight but when only counting first play through to defeating final boss this games wins. There a interlude in the middle of the game that probably more hours into the game then most JRPGs take to finish.