Probably Brawl. Played the hell out of it when it first came out, then played it with friends for a bit pretty much every time they came over (a total of like four or five times, damn them living so far away and being busy all the time :/), including playing up to the Great Maze on Insane in one sitting (and screw the SSE haters, that was fucking awesome).
In the end, I really didn't play it all that much. I might have actually spent more time with something else. It's a shame, though. If the online mode was actually playable, it would be my most played game by far.
EDIT: Actually scratch that. I just checked. Over 65 hours of play time and 135 hours of power time. Definitely my most played game this year. Though it would've been way more if the online mode was actually decent. I would seriously kill to have just a rerelease of the game with a functioning online mode and WiiSpeak support.
I do kinda wonder how in the hell I got almost twice as much power time as play time.