The lightest/coolest one that I have found so far has been the Kool Coat Airstream, which I chose because I needed the UV protection on my gray. It is great at protecting the coat, cool—ish (I live in NC where the summers are swampy with heat and humidity). He still sweats, but not as much as the others that I have tried. The quality isn’t particularly good, the mesh seems to rip easily (he is a OTTB, and not easy on the blankets, and he is out with my chestnut monster mare, who keeps him well in line with her teeth, LOL).
Another one that I think is cooler for the horse is the Schneider’s mosquito mesh- it is a heavier mesh that sort of stands away from the horses body a bit, but I hate the fit for my horses, and it seems to bunch in weird places and get massive wrinkles that will not come out, even with washing.
I would love to hear about other options that are cooler- between the humidity, heat and insects here I end up just keeping them inside during the day, but even the nights can be pretty uncomfortable.