For me Wii Sports has been THE defining game. My first, and only game for a while, showed me the potential of the wii and wiimote. Simple movements in baseball and bowling seemed shallow at first, has provided me with hundreds of hours of enjoyment.
Another defining game, although to a smaller extent, was Metroid Prime 3. As someone who has had little experience with the Metroid, and Metroid Prime series, I was going to pass this one as well. However, watching the first hour on youtube, and seeing the on-screen cues which showed me that wiimote movement led to in-game movement, sold me on purchasing the game.
There have been other great games (Zack and Wiki, Trauma Center:NB, SMG, Zelda:TP, etc.) but none of these games have made as big an impact on this generation.
