Guitar Hero III had a hold of me for a while, I got that game along with Umbrella Chronicles and BWii... the two latter games have yet to be beaten (BWii has yet to be played...)
Fire Emblem: Radiant Dawn had me in a strangle hold until I beat it (3 days, 1 weekend, pure sweetness)
Melee and Brawl are obvious. Ever since FE came out, Brawl has been in. (Reason: I have no money for Mario Kart or Boom Blox)
Pokemon Diamond: This was the first game in which I finally understood how EV's and IV's work... so I trained my team with that method... and it took a very long time...
Sonic & Knuckles: before I owned the game I rented it every weekend and I played through it religiously. My parents finally wizened up and bought me the game.
Warcraft III: I still play that game to this day (Custom Games ftw) Mauls are still a Smeags favorite, although everyone plays DotA now-a-days...
