Fed is fav, but there's a question mark behind his health/form.
Nadal might play under painkillers, but if he needs much of them, his feel and thus movement will suffer.
as for the WTF chances of the other players:
a rested, healthy Cilic definitely a threat and signs of peak Cilic about ready to appear.
Cilic has never been a really strong indoors player (and Fed usually deals with big servers pretty well btw).
a Cilic in top form would still look quite dangerous, but he's played a lot during the last weeks and the whole season. one may think that he got sufficient rest after Bercy, but i wouldnt be so sure. the whole weight of the season may be too heavy in the end.
No doubt Zverev can beat Federer, but the Zedbot not wholly comfortable on these surfaces
quicker indoor conditions tend to make Zverev less dangerous.
because of his clutchness and the level of the opposition his chances still look normal, but hardly better than those of most other contenders.
Dimitrov in fine form and managed to get sets in early 2016 and a much better player now.
he has very weak nerves and is so likely to falter against Fedal.
against the younger guys (than Cilic) he seems more confident however. anything possible there.
Thiem was in much worse shape last year [...]
my best guess is part of the current demise is mental.
after Vienna he said he has a smaller chronical physical problem
and this also makes more sense with respect to his slumps this season. certainly not just mental.
anyway, he has the game to beat all these guys (bar healthy Fedal) at the WTF, and none of them looked really strong of late either.
Goffin:
solid. if the others fail to set the bar high, he can be dangerous too. hard to see him going all the way though.
Sock:
got very lucky at Bercy. this limited player is hardest to imagine to be a danger at the WTF.
then again, the gap to the others is not that big really, so if he has a great run, who knows.
summary:
behind Fedal (?) this WTF is extremely wide open.