Cobblestone and Wood are the most common choices, as they are readily available and easy to farm in quantity ... you should link a Nether Portal to your mob spawner. Alternatively, you can ...

You have to mob-proof an entire Nether Fortress then kill Wither Skeletons using dogs for this farm, which sounds like a lot ...

Additionally, magma cubes are the only commonly spawning mob in ... you find a Nether fortress within a Soul Sand Valley, it can easily be turned into an efficient wither skeleton farm.