Oh God, it would have to be Warhammer Online. I've played almost continually since it's release in 2008. There are only a handful of addicts like myself left playing.....still searching for that perfectly timed rotation and elusive killing blow against some smart ass kid with a bunch of fancy pocket items. I unsubbed two days ago. We'll see if it lasts.
I've put way more hours into Plants v Zombies than any popcap game deserves.
Back in the day, I loved the Exile games. (I played I, II, III and maybe IV).