This is a bit like asking which car is best for driving. If we make the reasonable assumption that people buy what they feel is best for them, then the answer can come from market share. WebLogic and WebSphere are currently running neck-and-neck for leading the market share. JBoss has a loyal following, largely because it is free. JRun, Borland App Server, and Oracle 9i also have a number of adherents, as do something like 20 other app servers.
I always see weblogic and websphere and since I've been doing j2ee that is all I have worked with professionally. Personally, I have checked out JBoss and JRun and some otheres. I like JRun quite a bit and you can download a developers version for Windows which can do anything the Enterpirse edition does but it only accepts requests from the machine that it is installed on.
No more rhymes! I mean it!<br /> <br />Does anybody want a peanut?