To the older bearded gentleman in the Beemer in front of me at the drive-thru at McDonald's (no, I'm not proud of it):
1) I really don't have time to explain the concepts/gestalt of
a) FAST food
c) Talking to a minimum-wage employee through a small speaker box
d) McDonald's, in general
so you will just have to take my word for it that in a lot of ways, "Which of your sandwiches has the least amount of calories?" is not an appropriate question for this particular time and place.
And, 2) after the extremely patient employee, who has infinitely more self-control than I would have, calmly explained that yes, McDonald's actually has a written policy that they cannot provide that information specifically over the drive-thru headset, but that he would happily provide a pamphlet for you at the window, and you decide to throw caution to the wind and just wing it with your order...I think it might be safe to say that the Ranch BLT Chicken sandwich is probably NOT the sandwich with the least amount of calories.
In general, when looking to lower your caloric intake RANCH and BLT are two words to steer away from. In addition, McDonald's as a whole might be something to steer away from. These are just some thoughts.