Blackstone and nether gold ore, both for lore reasons. Aside from looking gorgeous, Blackstone explains where the wither skeletons get their swords from, and the same goes for nether gold with the piglins' gold gear. As of 1.16, the weapons used by nether mobs and the materials in their environment are perfectly balanced, as all things should be. I also really like crying obsidian since it was originally added way back in indev.
