The absolute best way to hide the fastener head is to use the Cortex Concealed Fastening System by FastenMaster or the Pro Plug System by Starborn. 

Both systems utilize a countersunk trim screw and a plug made from BOTO. 

If you are painting the BOTO trim, CrackSHOT by DAP is an excellent high performance spackling paste that is also a very good nail hole filler.

If you don't intend to paint the trim, use one of the screw and plug systems mentioned above.