Any Zelda Game, Mario Party 8, Mario Kart Wii, Mario Party DS, Final Fantasy-Chocobo's Dungeoun. For me the tie in Zelda Games is between Twilight Princess and Majora's Mask. Majora's Mask wasn't that hard for me, but still hard.
Use to be WoW, but not it's probably Team Fortress 2, with Left 4 Dead at a close second.
My all time favorite game probably to date has to be Diablo 2 though. I still yern to play it these days. Just need Diablo 3 to come out and end my social life like D2 did.