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.
Good advice from both above, so let me come at it a different way.
Used to be you'd come here for a psu rec, and you'd be told Seasonic, PC Power & Cooling, & Antec Earthwatts (not all Antecs). Because these were the only BRANDS that were KNOWN to consistently produce quality units across the power/model range. The Corsair got added to that list.
But there are several models branded OCZ and others that are very good units. They are believed good because they are reviewed by respected reviewers who actually do more than measure the cables and comment on the cool paint job.
So, I go to Newegg knowing the size (eg 400-500W) I want, and sort by price. Discard all units under $40 (unless I recognize one). Look for the cheapest one of the above-mentioned brands. That's my psu, unless I can go cheaper, google the brand/model, and find a quality review. Often the $ difference is small.
Then I go one size up (500W-600W in this case) and repeat the process, mainly to see if one is on sale.
In the case of Antec, I am open to models other than Earthwatts, but I look for a review. Lots of reviewers choose to review Antec.