Helmet. If I'm going to push myself on the MTB then i'm going to take an occasional ender or meet a tree--and helmets are damn handy when that happens
And ever since some %^$! left turned into me on a road as a teenager (no helmet, though happily no injuries) I'd never road ride without one. It's not me that I'm worried about, it's them.
If I were on a bike path, no cars, mellow ride... maybe I'd skip it. But never if my kids could see me (yet another reason to wear one from my end)