F-Zero.
GX is the pinnacle of racing games to me. As much as I enjoy kart racers and simulators, GX provided me with the most fun I've had racing and some frustrating moments. I'll say it again, the F-Zero Universe has so much potential to expand into other genres. They created 30+ characters with a personality of their own, just imagine...
I also hold Custom Robo, Glory of Heracles, The Legendary Starfy, Wave Race, and 1080 Snowboarding dearly.