Don't get a hat that seems too stiff - it will sit oddly on your head.
Don't get the trucker mesh hat - the early 2000s have come and gone.
Don't get a hat made out of leather/suede/fur/felt - stick to cotton or a cotton-poly blend or linen.
Don't get a New Era flat billed baseball caps - those are ridiculous.
DO get a hat with either an elastic-fitted band or a velcro band - avoid the plastic snap kind.
DO get a hat that is neutral in color and logo - unless you have a hockey club that you support.
But, avoid giant logos or large embroidered logos.
For a plain ball cap, I'd go with one of these
Amazon ball cap