So you weren’t satisfied with my “they’re all the best” answer up top, were you? OK, if forced to pick just one virtualization application to use today, I would pick (narrowly) Parallels over Fusion, with VirtualBox in a more-distant third spot.
Parallels is the undisputed speed king, it supports a wide variety of Windows games (an avocation of mine), has OpenGL acceleration available almost everywhere, and offers a ton of features. However, that feature load makes it somewhat more complex than Fusion, and its Coherence and Crystal view modes aren’t as well integrated with OS X’s windowing system as is Fusion’s Unity. Also, you’ll need to do more work to experiment with new virtual machines, beyond the 87 provided by Parallels.
Fusion does have a clearer interface, and that huge library of virtual appliances; it isn’t too shabby at gaming, either. Still, when I consider overall performance, feature set, and 3D gaming support (probably the tie breaker), Parallels 5 is my pick as the cream of the current crop—but just barely. Both are excellent four-mice programs; you won’t go wrong with either choice.
As I said last year, “I fully expect that the next round of updates to these three programs will both address current issues and bring exciting new features to the table. A little competition is a wonderful thing!”
Overall, these are three very good programs, and each has something to offer to prospective users. Take your time, do your homework, test drive all three, and go with the one that does the job you need it to do best.