Personally the first MMO i ever played, I found quite addictive. Runescape. But since everyone's played that and its so outdated to everyone except newbie gamers, its not really any good. Uhh maplestory is addictive for a while, except community sucks and the grind gets unbearable. Trickster is my favorite for addictiveness. The only problem is the quests die off after 30+ so I'm just chillin until they release those so I'll have soemthing to do on that game besides level up. It's overall fun but nothing other than drill & fight really. I myself am looking for a real simple addictive game, or something else (check my post).
