...not Barcelona, I think. Despite being a Barca fan, I'm realistic enough to see that out of the three Spanish teams competing Valencia probably have the best shot at it.
Why? Real and Barca have both proven that they can't beat quality opposition this season, at least so far. If that doesn't change soon, then obviously they'll be knocked out. Valencia on the other hand have gotten good results against the likes of Barca and Roma, are in great form and probably feel unbeatable right now.
The counter-attacking tactics employed by coach Quique Sanchez Flores are well suited to a knock-out competition, and their water-tight defence should stand them in good stead to advance far.