I can't make a personal recommendation other than to suggest you avoid anything described as "small crown", you'll likely end up with barely any material to try and drag in the direction of your right ear (I speak from unfortunate experience).
SD hats and chip bags have their place, but unless one is an AVM or above; in a position to avoid wearing No3 Service Dress; or simply choose to ignore AP1358, then all currently service RAF personnel should possess a beret as it is the only currently authorised headdress (other than a helmet) when wearing Operational Clothing. Public service announcement over.
