From what I have seen the OpenPilot boards seems to be the cutting edge for multi-rotor (or anything else) control.
32bit, highly configurable and fast. The project is a labour of love and is under a Creative Commons non-commercial license. This is great but it makes the boards somewhat hard to get hold of. From the openpilot store you can commit kickstarter style to go really bleeding edge with the new Revolution (read up on it first) or wait for a batch of the proven CC3D. The other option is to build one yourself which I think I will attempt once I get to the limits of the KK2 capabilities. Mochaboy built his own and made this really great video:
To see how they perform in the hands of a top pilot check out Juz's channel. This is my favourite:
RCHacker blog >