Foundation because my skin is not perfect. I could live without other things I guess.
I stopped using eyeshadow & liquid eyeliner I only use mascara when I go out (I very rarely use eyeshadow & eyeliner).
But my lashes are dark and long (I used to cut my bottom lashes a bit lol, because I also had liquid eyeliner on my bottom lid... and my lashes were on my way) so I'm not even bothered if I go out without mascara.
So I can't live without foundation I guess. I never leave my house without it.