For me, it's always an alcohol! Needless to explain its importance! Shampoo and conditioners of course are just as important to me too but I always make sure to keep those in big bottles so they could last me for at least a month.
I'm really not big on cosmetics and other beauty products. I don't even wear face powder on ordinary days. Not that I don't have any but I really don't feel like using them when I don't need to. But show me where nail polishes are and I will surely be right there and then. Unfortunately, I don't think I have any uploaded pictures of my nail polish collection that I can pull out from my hard drive other than this (it's blurry, sorry) and in which I actually didn't buy myself, hehe. They are gifts!