Most of the close to town stuff is going to be up near horsetooth res. Maxwell and Foothills are on the east side of the res so a little more accessible, but that means also busier.
For my money, I’d go over to the west side and pick a route. I usually get suckered into riding up Towers which is a solid test of your fitness. For down, it’s usually Mill Creek. Frankly I don’t remember anything on Mill Creek that earns a black diamond in my book but I was raised on CO jank and have a Ripmo.
Are you a Colorado intermediate or a flat lander immediate? That might be what would be my deciding factor.
Another option is to go south from town on Blue Sky to Indian Summer. Both are lovely rides but they can get hot and don’t have a lot of shade.