The original Geometry Wars was one of the purest, most exhilarating gaming experiences of the post-arcade era. It combined the visuals of Tempest with the frantic, twin-stick shooting of Robotron 2084 to form a thoroughly modern point-driven shooter. The Wii exclusive sequel Galaxies added level variety and the great risk/reward mechanic of collecting geoms to increase your point multiplier. It's still the game that I play the most on my Wii.
The third game in the series builds on the Galaxies formula but with the mind-blowing twist of moving the game grid onto curving non-euclidean surfaces. The result is nothing short of spectacular.