Valve cover gasket opinion...Cork vs Rubber

I've always had decent results with cork.

All mentioned above, move around the covers with a small hammer and make sure all holes are flat, clean cover well and apply some RTV (doesn't require gobs) install gasket on cover and let set up overnight allowing for the gasket to "stick" to the cover.

Make sure surface on the head is clean and there isn't old gasket material stuck.

As a final step I use my finger and smear a little grease all the way around the gasket where it mates to the head, reason being if you need to remove the covers the grease helps keep the gasket from sticking to the head.

DO NOT over tighten.

I have removed covers and reused the gasket in place several times without a leak. YMMV