My personal preference in a cap is a Flexfit. The edge band is elastic so the fit is secure, comfortable and quite literally made to measure to your specific cranium. They tend to come in two sizes and being elastic I could imagine some people on the very edges of the smallest or largest size limits may have issues, but I think the vast majority of folks will find the fit to be excellent. Also the options that include a majority mesh construction are superb in warm weather, as you might imagine.
There are a lot of options out there for Flexfit caps in military colours and camo patterns, however most of them are only trying to fit in to the fashion market. They almost always have top buttons which are insanely uncomfortable underneath the headband on hearing protection. RE Factor Tactical offer these caps in all the most popular tactical flat colours as well as Multicam, Multicam Black, US Woodland Camo and Tiger Stripe as shown here.
When compared to most civilian options for equivalent head gear, the top button is removed in favour of a very small loop field and further loop panels are added both front and rear in order that patches can of course be added if desired. There is also orange signal panelling on the inside along with RE factor info for some different explosives, handy for some EOD types.
Most retailers of these are charging just under $30 and I have to say I think that’s a good deal given the comfort and features these provide when it comes to wearing with other tactical type kit, whether your use is professional or merely recreational.