I'm surprised nobody mentioned brubeck (http://brubeck.io) until now, which takes a completely fresh approach to concurrent handling via implicit context switching (more on their website).

Obviously, as idan mentioned already, there is no single "best" framework, so it really depends on the type of application you want to build whether you should choose something like bottle/flask, django/mezzanine/pinax or brubeck/tornado, or even twisted.