I am undecided between lipgloss or powder, I do like to blot my face with a bit more powder every now and again, yet I always like to keep my lips nice and shiny! Between the two I would probably have to say powder, as vaseline works wonders on the lips and that isn't really classed as cosmetics!
Mascara - definitely. Although I've got brown hair my eyelashes go almost white towards the end so it looks like I haven't got any. I'm currently using Max Factor's 2000 Calorie and it's ace.
i'm assuming you mean one that we'd use if we could use only one as opposed to the one we have to keep with us for emergencies once we've gone out having done our make up...if lipbalm dosen't count (as if you wear loads it looks like lip gloss!) i'd have to say a black eyeliner/brow pencil. foundation is evil!