Mine is a Boxster S
I can kick-in some info on this since I've owned a Boxster S for three years....
Unless you're the queen/king of whatever, it's hard to compare a Porsche to the best-of-the-best. From my personal perspective, my Boxster is a dream come true. I've owned three different Alfa Romeos, a very old Triumph (TR-3A, still have it), and have driven various BMWs, Jaguars. etc. But The Box is the best performing car I've ever owned. All of the power I could ask for, handling from heaven, great looks, and a surprising amount of trunk space.
However, I test drove a '04 M3 with SMG yesterday and I'M SOLD. Haven't checked out the super exotics, but if given a choice between a Boxster S and an M3, I would pick the M3 on performance alone. Never mind that it seats 4.