So comments on another thread lead me to start this one...
I'll go first, My favorite era would be the period from about 1902/3 up to the late 20's, say 1927. Why? Because the narrow gauge was a vital, profitable operation. of course the profits were mostly being drained off for other parts of the system until the era ends in a burst of catching up in the 20's.
Why starting in '02? the end of service to Denver set the operational pattern that would last into the 30's and saw the end (other than the Montrose - Grand Jct line & North Fork branch in '06) of the transition period away from the transcontinental era that had begun in 1887.
Why ending in '27? Because that is the year that the Salida-Alamosa passenger was bustituited which to me signals the start of the contraction and deliberate downgrading of the narrow gauge, the final decline that ended in '68.
hank