New Super Mario Bros. Wii
Let's not kid ourselves: running and jumping is fun. In terms of mechanics, few things are inherently more enjoyable than simply leaping past obstacles, making them a non-factor through a combination of agility and wit. New Super Mario Bros. Wii is all about running and jumping, and because of that New Super Mario Bros. Wii is inherently fun.
I have said this before but it bears repeating: World 8 in this game may be the best and most pleasant collection of platforming levels I have ever played, a perfect balance of frantic difficulty, adequate visuals, and near-death moments that can only be gotten past with courage, speed, and a keen eye. As a platformer, New Super Mario Bros. Wii is almost entirely without peer, and this alone would earn it a spot on this list.
But it's also co-op, and I am a co-op junkie. It's wonderful in that it allows players to support each other as needed, but does not too greatly punish attempts to sabotage one another. The fun factor of running through the game is increased almost exponentially by playing through together, and with a game as fun as this one that is saying an incredible amount.