Java EE (Java Enterprise Edition) General Discussion

What is the best servlet engine in terms of robustness and ease of use?

I've downloaded the Sun Appserver, which I'm lead to believe is an all-in-one solution (server with servlet engine), or do I have it confused with the Sun Web Server? I've also downloaded ServletExec, which runs with IIS. I've notice there are nearly a dozen alternatives. It is a bit bewildering, I suppose that I should just pick one and go with it, but I'm sure thre must be advantages and disadvantages with so many choices.