How do you go about determing what writing sample to give to an employer?

I was asked by my (hopefully) future boss to email him a writing sample.

I have narrowed it down. But when I know someone is going to be reading my work I start doubting myself.

I am going to give him a paper I wrote in school, so it is an academic piece. But what types of things do people look for when reading a writing sample?

What is your process for determining what to hand over?