there alot of virtual machine to emulate mac os,which of them is best?

Do you want to run a Apple OS as the Host or Guest (or both) machine? Also what version OS are you refering to?

VMware Fusion and Parrells are popular.
http://en.wikipedia.org/wiki/VMware_Fusion
http://en.wikipedia.org/wiki/Parallels_Desktop_for_Mac
http://en.wikipedia.org/wiki/Comparison_of_virtual_machines

Not really related but OSX Leopard 10.5 now has BootCamp for 'native' dualbooting.
http://en.wikipedia.org/wiki/Boot_Camp_(software)

As for 'emulating' Mac OS first thing I think of is Mac-on-Stick using Mini vMac.
http://portableapps.com/apps/operating_systems/mac-on-stick

or the OSx86 project aka Hackintosh...
http://en.wikipedia.org/wiki/OSx86