Depends on what you mean by "big", as well as by "city". The biggest city proper, other than NY, in the NY-NJ-CT tristate is Newark, NJ (275,000), about 25 mi west of NY. As far as other large metros go, Philadelphia is about 90 mi to the south west (pop ~6,000,000) while Hartford (1,200,000 or about the size of Buffalo) , Providence (1,500,000) and Boston (5,500,000) are about 100, 150, & 200 mi, respectively, to the Northeast.
If you look outside of the Denver Metro area (which includes Boulder, Castle Rock, Thornton, Commerce, Arvada, Aurora, etc.), the nearest city of 10,000+ would be Limon.
Denver and Colorado Springs.
Denver
Denver Colorado
Denver and Colorado Springs.
Denver, Colorado Springs, and Pueblo
Just one, Denver.
Denver, Colorado Springs, and Aurora.
The 5 largest cities in Colorado by population are Denver, Colorado Springs, Aurora, Fort Collins, and Lakewood. Denver has a population of 600,158. Colorado Springs has 416,427. Aurora has 325,078. Fort Collins has 143,986. Lakewood has 142,980
They are the most populated Colorado cities.
Consider Denver.
Denver International Airport - Denver, CO (DEN / KDEN).
The closest international or regional airport is Denver International Airport (DIA).