Since there is a lot of good models out there, it's difficult to say what is the best one. It depends on your needs, but the Enermax Modu87+ is, without any doubt, one of the best PSUs ever made. The Seasonic X-560 is pretty new so there is not any qualified review yet, but the first impresion is very good.
The Cosair VX550 or the Antec TruePower New 550W are the best bang for the money nowadays.
You will find usefull information and qualified reviews about these and many more power supplies in the PSU Review Database from RealHardTechX: http://www.realhardtechx.com/index_archivos/Page541.htm