I don't play any MMOs right now as I can't find one that I really like.
although I did used to have a serious WoW addiction, and I mean serious. I played 6-8 hours every single day and for close to 4 years straight!! I even cancelled family things and appointments to play. it was very very stupid but its incredibly hard to get off but thankfully the community became so repugnant and vile and blizzard kept adding things I loathed (ie achievements, pet battles etc etc) so in the end it wasn't that hard to quit.
I have played many mmos. Ultima Online, Everquest 1-2, Guild Wars 1-2, LoTR, Star Trek online, Star Wars etc etc but thankfully none were ever as addictive as wow was.