Probably Audiosurf. I will always champion Audiosurf. It's just pure and simple fun, plus you're listening to your favorite music at the same time. It can also be intense as all hell with fast or complex songs, which gives you that old-school rush that arcade games used to give you decades ago.
I've told this story before, but one year my sister and niece were staying at my house over Christmas. That was at the crest of the Wii craze and that's what she got for Christmas. My family was here, about 10 people for our annual party. They started on the Wii, but I had my PC in the living room at the time and I fired up Audiosurf as an extra activity. Within a short period, everyone was gathered around the PC and taking turns on Audiosurf while Wii Sports sat there unused.