Probably one of my favourite Wii games, New Super Mario Bros. Wii brought this generation some great 2D Mario & crew platforming action.
Fun playing alone is a given with a Mario title, but what sets this one apart is the ability for up to four players to play at the same time on the same screen. Lots of LoL's, frustration, and thrown wiimotes are guaranteed and within 30 seconds you will know who the weakest player of the pack is (they'll be the one holding everyone else up.)
But, when two or more players of good skill level play together, it's magic. Just check out some of the Super Skills videos on Youtube or in the castle located on the first map screen (talk to Toad and he'll get you set up.)
The game is classic Super Mario Bros.: it's challenging for players of all skills, but only if you try to do anything more than simply complete the map. Collecting all the star coins, doing the levels within a time limit, or collecting unlimited 1-ups are all really tough and require a lot of practice.
If you haven't played New Super Mario Bros. Wii, you definitely should pick it up and enjoy it.