A black BDU cap. Tan if it's a hot day.
Sturdy, $8 for actual surplus ($45 for a GAP hat? No, thanks.), comfortable. I tend to wear it a size larger than a soldier would (to fit hair), so the brim's low enough that it really does stop shells from falling onto my face.
Also, there are literally zero pictures of me above drinking age without the hat, and the majority of my friends have never seen beneath it.
If I ever buy a Thomson, then I'll get a fedora to go with it.