If I could only use one piece of makeup for the rest of my life, I'd be in trouble. I don't even ever want to have to think about it. I don't have a choice in saying concealer, because my circles are so bad.
Actually, I'm going to say Diorskin Airflash... that stuff makes my skin look flawless, and actually does a great job hiding my circles. So there you go... one product with two uses.
If my circles weren't so bad, I'd say mascara.