It depends on what time of the day it is...
Early morning, before parks open, I would walk out GCH to DTD and then head to the esplanade to enter through the main gates. The GCH entrance to DCA is too crowded at park opening and it takes longer. And if DCA isn't open yet, it's not an option, anyways.
Later in the morning, or if you want to see DCA on the way, go through special GCH entrance into DCA, then through the gates, across the esplanade and into DL.
Other times of the day, the monorail might be best if you plan to do some things in DTD first...but that would be my last suggestion as you have to backtrack to the monorail, compared with just entering the esplanade.