Depending on how much you want to spend, I'd suggest either a Rambo Protector Fly Sheet or an Amigo Fly Sheet. I have used both of these sheets extensively and LOVED both of them. The Rambo is definitely sturdier, so if your horse does end up being pastured with rougher playing buddies that may be a better choice. My mare is quite easy on her clothing and her Rambo sheet lasted at least 8 summers. The past few years I have been using the Amigo sheet on my mare and it has held up very well. It is softer material than the Rambo, therefore not quite as durable but seems like it might be more comfortable? Summers here can get pretty hot (90s) and humid and I would occasionally see sweating under both sheets, but not often at all.
I have also used the Amigo Mio fly sheet but didn't care for that one as much. It got a lot of little snags in the fabric and didn't fit as well in my opinion.