Best by what criteria?
In my personal experience to date:
For power & flow & low-noise, as of last week, the
Swiftech MCP655
For pure efficiency, the old Swiftech MCP600. However, if you feed it 14v then ignoring the extra noise, it's still better than the MCP655 when one factors in heat dump.
For compactness, low-noise, adequate flow and low noise, the MCP350.