What are the chances Del Potro beats Nadal? Is Federer the favorite?
Del Potro could beat Nadal. But probably not on these courts. Simon gave Del Potro enough trouble with good defence, so Nadal should be able to beat him easily enough. These courts don't give Del Potro quite enough height to just attack off as many balls as he'd like to. But, that big serve, and an ability to attack Nadals serve, could trouble Nadal.
Federer is the favorite on paper, absolutely. The number 1 seed. Won it last year. But if Cilic was healthy last year, he probably would've beaten Federer in last years final.
Federer is clearly not as good as he used to be, but so good that still lets him keeping winning slams. He started winning slams again because he switched racquets. Switching to a bigger head, makes his backhand so much more solid, less of liability. But his serve is also not as accurate and deadly as it used to be. That will be a problem against Novak or Nadal. Although on balance it seems to have made him an all around better player, as evidenced by this late slam resurgence. Particularly, playing with this new head makes him far better against Nadal, who focused on Feds backhand exclusively and easily. Basically you have a Fed who is not as sharp as usual, because of his age, but the new head has given his game a bit of a new lease.
Novak or Nadal could take down Federer for sure. Although, none of these three are at their absolute best, of course. Novak has been playing into form, too.