What are you gonna be running on that VM, and what are you gonna be running that VM on ?
In general, I would recommend Xen with paravirtualization if you have an x86 system (will not be enabled if guest OS does not have modified kernel, i.e. for M$ Window$). If you have SMP, do not use virtualbox or vmware player, because they don't support SMP. If these don't work for you, then try vmware ... or others.