Which is futher away will depend on where you are starting from. Compared to each other, Denver is further north and further west and Dallas is further south and further east.
denver
Oklahoma City is further from Dallas than Dallas is.
Dallas is not a capital city. (In Texas, it's Austin.)
Dallas to Denver -- about 800 miles Dallas to Salt Lake -- about 1250 miles
Oklahoma City is north of Dallas.
Dallas, Darwin, Denver
Amarillo, TX
Denver, Dallas, Duluth, Dyersville, Destin...
Detroit , Denver , Dallas ,
Kansas City and Denver
Dallas, Detroit, Denver
My guess is Oklahoma City because Dallas is farther and i think Denver and Albuquerque may even be closer to Amarillo then Dallas is.