Try to pick maps with as many different biomes as possible, you can get your hands on allot more stuff that way.
My current map has the first 2 levels full of white Sand, level 3 is Obsidian heaven, 4 is Limestone heaven along with some bauxite (no magma in the area so it's useless in this case), and level 5 has a nice amount of magnetite and malachite.
The map is peaceful though, maybe I should be grateful, but the last map with the giant spiders and hordes of undead was pretty intense