What's the best air filter and where's the best place to buy?

[Shifu]

I use Pipercross filter after some research on filtering performance of various brands. The foam type filters outperformed the standard ones with fibre media (including OEM) every time. That means in filtering performance, not in gaining horses. It is an old catch 22, either good filtration and some restriction of the air flow or high flow rate with crap going through to the engine. Racing bikes do not use air filters at all as far as I know simply because they are getting rebuilt long before they have a chance to wear due to air polution.
K&N is also a foam type so it probably is good as well.