Are you asking which is the best J2EE application server?
If you are, you won't get a straight answer as it's mostly a matter of opinion and varying requirements.
The biggest names are BEA WebLogic, IBM WebSphere, and
JBoss. WebSphere is a bit behind in the latest specs, WebLogic (as well as WebSphere) cost big bucks but scales well, and JBoss is open source but performs well too.
Then there are Caucho Resin, IronFlare Orionserver, Oracle 9iAS, Borland AS, Macromedia JRun, Pramati Server and others that don't have so much market share.
You'll probably find your favorite EJB implementation among these, but the only way to find that one is to give all (or most) of these a chance.