Best head gasket sealer to use on a motorcycle?

Dont use silicone on motorcycle gaskets, it tends to extrude internally when you tighten up the bolts. Bits of this extrusion can break off & block passageways. I always use Permatex No3 on all faces during a rebuild. It has a number of benefits. It does improve the sealing on slightly less than perfect joints & come teardown time again it stops the gasket material sticking to the flanges & having to be scraped off potentially doing more damage. Cleanup is simple with metho ( ethanol)

Personally I'd rather reuse a good gasket with Permatex than a new one without (except head gaskets & exhaust gaskets of course)

Head gaskets do not usually require a sealant.

None. Both cam and head gaskets go on dry

For a dealer - I went to the local Honda dealer for that head gasket (and little O-rings).

For a sealant........proper torque on the bolts worked just fine.

I use RTV blue. You can get it at any auto parts store.