The alphamegalog of the cities of Colorado lists the largest city in Colorado for each letter of the alphabet. In this case, ‘largest’ is based on population within the city proper.

Aurora
Boulder
Colorado Springs
Denver
Englewood
Fort Collins
Greely
Highlands Ranch
Idaho Springs
Johnstown
Ken Caryl
Lakewood
Montrose
Northglenn
Orchard Mesa
Pueblo
(no Q)
Rifle
Sherrilwood
Thornton
Upper Bear Creek
Vale
Westminster
(no X)
Yuma
(no Z)