Mine would be the shoreline trails around horsetooth reservoir, I live about 1 mile from the foothills so this is the easiest to get to. Maxwell, the trail behind the CSU stadium, is kind of tricky for me, but once you get up to the dams road (centennial), the trails are pretty fun from there. My favorite loop is to head north on shoreline and then end up on the trail that descends to michaud lane … I don’t know the name of that one. Then just cruise home on Overland Trail (actually a road).
Or descend toward West Laport on any one of 2 trails whose names escape me to my office, a nice mtn bike commute in the AM! Just not for the mornings when I’m carrying my laptop 