what MAC?

Ok, I have a question. As a MAC cosmetics virgin, what is the one product I should buy? What is the one MAC product that you use all the time, that I should definitely have in my makeup bag?

It can be anything at all from their collection. Here's some ideas from http://www.maccosmetics.co.uk/:

Please leave me a comment, to let me know. The most popular answer wins, and I will go out and get my first MAC product!