As of this moment? Or overall? The question shifts based on time focus.
I'd say that C9 remains at the very top as of this moment, the recent loss to Dignitas besides. Your informal poll'll say otherwise, of course - witness the raw power of the TSM hype machine - but they were effectively untouchable all last year, and still remain the most North American team at their level of play.
But overall? That's a lot murkier. TSM's improved massively with the Bjergsen pickup. Dignitas finally looks serious again (though I'm not holding my breath - they had a sustained spike last year too yet couldn't hold it together). Even CLG showed that their strategic planning's been refined, if not necessarily their capability to execute. And down in the Challenger Series, LMQ's stomping boots are downright muddy with the viscera of their victims.
Who is the best team across NA? Ask me again at the end of summer.