Virtual Box does the trick for me, runs great, though often a new version comes out, for which extensions need to be loaded seperately (installed machines keep on working though).

Virtual Product
 

Virtual box.

Virtualbox
 

I have to admit I haven't tried the others, but VirtualBox has been so reliable and easy to use, I haven't had the itch to try any of its competetors.

Wrong. Proxmox is a Debian-based distro with a Red Hat-derived kernel and a suite of VM management tools.

I would have to vote for VMWare... It is far more robust, and supports hardware forwarding MUCH better than Virtualbox does... You can do a lot more cool tricks with VMWare than you can with Virtualbox. Also, VMWare has tech notes about most of these tricks... Like booting another OS on your physical system so you don't have to reboot to get to your other OS, or simultaneous input from two separate systems allowing one computer to split into two (Or more) separately controlled systems with independent input. As well as booting network drives, or countless other tricks...

Virtualbox can't do these things.

VirtualBox for me.. :)

VirtualBox.

KVM-Qemu on Slackware, Xen on NetBSD