It's fascinating that you can get to the level of atomic material properties as a spinning top hacker. Diamond seems like it'd be the obvious winner, if you could somehow get a perfectly polished and smooth surface.

I'd love to see a small Prince Rupert's drop for a tip and a ruby/sapphire spinning surface - you'd need to make a ton of drops, probably, but having a round, nearly spherical contact geometry and super smooth surface seems like a winning combo.