VirtualBox - decent application, less features and performance but its free
Parallels - best performance, typically has stability issues particularly with new updates
Vmware - Stable product, has similar features as Parallels but not on the same performance level as parallels
I like Vmware, in part because it has great tech support
I have used both VirtualBox and Parallels in the last 3 months...I like them both but as to which is better depends on your personal preference but have had no problem with either.
I am using VMs on both Windows and OS X host systems and it is nice having the same interface on both.
VMWare has always been the more appealing option to me. It is much more intuitive and 'Mac-like' than Parallels, which I find clunky.
Both those options are about $100 though, so if you're a casual user and don't depend on it (or require the premium features of the paid software), then use VirtualBox.
Does anyone know which works best with retina display? I have the 13" rmbp. I think both vmware and parallels support running in full resolution, but that doesn't interest me at all (way too small). Do either of them to a better job at "best for retina"?