Diamond is best, but new users have to be careful not to overdo it. Diamond can be very aggressive. Actually, any sharpening medium should be used gently, not pressing too hard, or the edge can be damaged. This is the biggest problem with those small, preset carbide sharpeners. Too many people think dragging the blade through them hard will sharpen it faster. Instead, it chips the edge.
Besides diamond, I also like ceramic, for touchups in the field and for finishing an edge.