By land area, the Phoenix metro complex is one of the largest in the nation. The great valley that hosts this giant city just happens to be surrounded by exceptional mountain biking in the great Sonoran Desert. The great South Mountain beckons to technical masochists. On the East side, the Gold Canyon serves up a variety of singletrack along with jaw-dropping scenery. To the Northeast, McDowell Mountain Park provides dozens of miles of quality trail, some of it purpose-built for biking, and to the Northwest, you will find seemingly-endless miles of cross country delight on the Black Canyon trail. While the summer heat is not just oppressive, but dangerous, you can ride here when the rest of the nation is frozen—and night riding remains an option in the worst of the summer heat.
Honorable Mention: Tucson
The Sonoran Desert extends further south through Tucson, which also sports plenty of excellent riding and even more, if you can believe it, of those giant saguaro cacti. Despite being further south, Tucson is usually a few degrees cooler owing to its slightly-higher elevation. The must-rides here include the truly world-class 50-Year Trail and the super-flowy Fantasy Island. If you want to ride through the densest saguaro forest around, hit up the Sweetwater Preserve