Tungsten Carbide is not a metal either -- it is a ceramic. Various carbides (tungsten, tantalum, boron) are extremely hard (near diamond) but brittle, as are other ceramics like cubic zirconia and corundum (Al2O3). Several research groups are trying to make them less brittle by making composites with carbon-based materials (graphite or nanotubes). In some cases, the nanotubes which bind up the grain boundaries to decrease brittleness also result in electrical conductivity (none of the carbides or other ceramics above) are electrical conductors so are not metals but the composites are conductive (although that does not necessarily make them metals).
Alloys are always going to be harder than pure metals. The hardness is determined not just by the elements present but also by the microstructure so second phases are used and grain size is kept small. Carbon steel alloy 1090 is normally quoted as the hardest true metal but it is also corrosive. To get really hard materials, people are coating metal parts with ceramics.