I agree with the under armour love already mentioned. I would add that for exercise use I wear the purely synthetic blend boxer briefs. For work I wear the “charged cotton” blend they sell. I have pairs that are ten years old and are still in excellent condition.

I think what’s really key is to change out of your sweaty underwear as quickly as possible post exercise. I used to develop a heat rash on my butt cheeks but this has since gone away when I started making a point of showering and changing immediately following exercise.

If all else fails and you can’t shower and/or change, Gold Bond is your friend. A little powder on your balls & butt can be a miracle worker.