Lipgloss. If I could never buy a new Lipglass or Dazzleglass or Lustreglass ever again, I don't know what I would do, especially since I'm allergic to lipglosses from a lot of other brands (NARS and Chanel being the two that spring immediately to mind). I'm a lipgloss fiend.
Second on the list is eyeshadows. A very close second, but I've only found one other type of lipgloss I like as well as MAC's, and it has a limited range of colors, and I can't live without lipgloss.