I used to use qemu on linux,
but it seems not functioning well on osx,
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If you are looking to game on Mac - use Parallels. Hands down it has the fastest 3D-acceleration virtualization on Mac.
If you need to run productivity apps mainly, then go with VMWare Fusion for its outstanding support and configuration tool, as well as superior OpenGL and productivity benchmarks.
If you would prefer free, then go with Oracle VirtualBox.
For a comprehensive review, you can read this article on Ars Technica.
I personally prefer VMWare, as it has a long history of outstanding virtualization products, and productivity performance is more important to me.
VirtualBox is currently the best free one. Commercially, in general VMWare has slightly better guest compatibility whereas Parallels is slightly faster — but you should probably ask a search engine about the specific hypervisor and guest OS you want to use, to see if there are any known issues.