By "best" probably wouldn't mean "potent" per-se...
There is a strain (mostly from the Cali dispensaries) called Hash Plant, or Master Hash Plant. This bud is actually so potent, that it will make you sick after just a couple hits of the bong. It's extremely heady and knocks you out like a light...
As far as overall quality, flavor, and consistency is concerned, there are a few that come to mind: White Widow, NYC/Sour Diesel, OG Kush, and AK-47. All of which are quite strong, yet have a manageable high, unlike Hash Plant.