Last time i meddled Alpha Bay was king. Seems like they are still doing strong? What is the general consensus on the way things are going at Alpha Bay?
AND how good is Dream market? What is its history and why has it gotten popular? http://www.coindesk.com/study-finds-bitcoin-has-a-new-top-dark-market/
I think "best" is subjective and probably depends on what's important to you. From a vendor's perspective, AlphaBay is an obvious choice, since it probably gets the most visitors and therefore exposes the most people to your product offerings. However, from a buyer's perspective, I'd probably go with Hansa. It uses a strict multisig system that prohibits FE listings, so every transaction is protected with full escrow. The multisig requirements keep everyone accountable, and in my opinion, this seems to attract a more professional assortment of vendors.
Also, Hansa's user interface and page loading speeds provide a much better "shopping experience" in my opinion. Using AlphaBay's search feature can be really slow and annoying at times, and their search results always seem to include listings of questionable relevance. Let me put it this way: if I had exactly 1 minute to find a reputable listing/vendor of ___ and I had to choose from Hansa, AB, or Dream's search interface, I would choose Hansa's. If I were casually window shopping for a new drug to try on...Hansa is like a strip mall in a gentrified neighborhood, while AB and Dream are more like open-air flea markets.
EDIT: I guess I should disclose the fact that I'm a vendor on Hansa. But my opinions here are very much genuine, and indeed the reasons why I chose Hansa. Also, after getting burned by Oasis' exit, I was definitely comforted by Hansa's multisig design, which virtually eliminates the possibility of Hansa running off with everyone's money.