#1 featherwood floors and walls
damage by getting whacked into something has something to do with density and weight, featherwood is the lightest thing in the game so its the safest material
A quick material breakdown, by name, density, and availability:
Featherwood -- 100 -- Good non-freezing biomes
Candlenut -- 140 -- Any tropical forest
Adamantine (raw or processed) -- 200 -- Any embark more than 2x2, most 2x2 embarks, some 1x1 embarks
Kapok -- 260 -- Tropical moist broadleaf forest
Willow -- 390 -- Around water, pretty much any non-freezing biome
Most other woods are 600 density, with some being a bit heavier or lighter. Glumprong is the only above-ground tree with a density of over 1000.
Subterranean woods:
Tunnel tube -- 500 -- caverns 2-3
Nether-cap, -- 550 -- cavern 3
Tower-cap, fungiwood -- 600 -- caverns 1-2
Goblin-cap, spore tree -- 600 -- caverns 2-3
Black-cap -- 650 -- caverns 2-3
Blood thorn -- 1250 -- cavern 3
So, in summary?Candlenut if you've got it, otherwise the common willow or cavern 1 woods. Featherwood and adamantine don't need to be remembered.