Which is the best servlet server or servlet engine (best performance, easiest, etc)?
There are a lot of products, and each has advantages and disadvantages. For a more-or-less full list of servlet engines and servlet-enabled web servers, see The Purple Tech Servlet Resource List.
The most popular servlet engines seem to be JRun and JServ. Both of these work inside the open-source Apache Web Server (easily the most popular HTTP server). Tomcat is the latest-and-greatest open-source servlet/JSP engine using code from Sun, Apache and JServ. ServletExec is reportedly very stable in high-volume sites -- but the other servlet engines are being used on some high-volume sites as well.