No blanket is the best blanket. Especially in the summer.
I always travel in a day sheet in summer and a woolen in winter, my horses normally seat up heaps so I can't have a heavy rug on them when travelling
My horses always travel in a fleece :)
A fleece is good because of its wicking abilities which helps the horse not get over sweaty as lots of horse get worked up and sweaty when travel
I would say i obviously depends on how cold it is but i would say a day sheet is your best all around option.