What Is The Best Application Server?

Naeem Bari asks: "I wanted to find out if Slashdot readers have a favorite Application Server. I want to compare products like Sun's iPlanet, IBM's WebSphere, BEA's Weblogic, Sybase's Jaguar et al. I am looking for comments from people who have experience using any of these products. What kind of features do they support (EJB 1.1, CORBA 2.3, XML etc.)? And whether they do these features really well or just pay them lip service? How about usability? Vendor support? Performance? Scalability? Failover abilities?"