There's no question. The best final I see possible at this stage would be Brazil-Argentina. Brazil because of expectations, and Argentina because, IMO, they're they NT that's been playing the best international football of the past couple of years, IMO.
It would be a nice rematch of the 2005 Confed Cup final, where Brazil limped in like a has-been, barely qualifying in their group over Japan. And Argentina was fierce... until meeting Brazil in the final, where Brazil suddenly decided they should play for real and thus destroyed them.
Oh, the rivalry...
(Though I know for many Europeans, the thought of a "provincial" South American final bores many to tears without their own kind...)