Well I'll tell you the games I got most addicted too.
Diablo 2, THAT is addicting, I played over 4 years easily. ( B2P then F2P )
Conquer Online, just unique and VERY addictive.. about 2 years now.
www.ConquerOnline.com F2P
Neverwinter Nights 1 and 2. B2P then F2P ( I'm currently playing the single player campaign of Neverwinter Nights 2, its awesome! )
Gunz Online, good gun game with levels if your tired of the big grinding and just wanna have fun, play this.
www.gunzonline.com F2P
EDIT:
Theres also World of Warcraft, but I got addicted till level 31 and stopped, sometimes I'm thinking of starting again though!