Well recorded vocals are a great way to evaluate the clarity and natural characteristics of your system. Some of them are also awesome performances. I love a lot of female vocals, but will listen to plenty of male vocals too.
Lately I've been jamming to a lot of Alison Krauss, Emmy Lou Harris, Sarah Vaughn, Mark Knopfler, Tracy Chapman, a little Enya, Willie, and even some Harry Connick Jr.