I don't know, maybe I'm completely overrating Nibali but I have the feeling he gets underrated. The giro field this year is great (not as great as it looked two days ago but still great) because 3 of the right now probably 5 highest rated gc riders are all targeting this race but that doesn't make Yates, Dumoulin and Roglic what Contador, Froome and Nibali were in 2014.
Roglic hasn't even been on the podium of a grand tour, yet people are hyping him up like a multiple tdf winner. Could we please not forget the last time Nibali rode a gt in peak shape on the last stage he finished, despite crashing, he was dragging that very Primoz Roglic up Alpe d'Huez. Could we please not forget that happened after Nibali looked way worse in his preperation races than this year. Could we please not forget he was in front of Roglic in the gc before abandoning? And how on earth are people looking at Nibali having his best preperation in years and use it as an argument against him. It would be more logical to say Nibali is back to his 2014 level than anything else looking at his tour of the alps. I genuinely think you could make the argument Nibali should be a bigger favorite than Roglic yet apparently Roglic is a bigger favorite by a countrymile. Just look at some of the odds the bookies give the two.
Then there is Yates who was obviously insanely good for most of the giro last year...before completely collapsing. Of course he had his Vuelta win later that season but was that Vuelta supposed to be the prove he is unbeatable for someone like Nibali? Like, I love Enric Mas, but Enric Mas could have been seriously dangerous for Yates without his off day at La Covatilla, so is Mas now supposedly on a completely different level than Nibali? Btw, it's also worth mentioning that the vuelta last year suited yates perfectly while this giro in my opinion doesn't (don't know how well he will cope with the really long and tough mountain stages). If Yates gets his giro legs from last year for 21 stages this time, I guess he'll be the man to beat, but I can't be the only one who thinks there is a good chance he'll only be in his vuelta shape and if that's the case, on this route, I'd put my money on Nibali.
Dumoulin imo has to be the favorite for this race because he has proven to be an elite gc rider for a few times now. I expect him to completely smash everyone in the San Marino TT and then it's gonna be very hard to drop him. That said, to return to the comparison with Froome or Contador in their best years. Dumoulin might be a great rider but he is not an Indurain, or and Armstrong. Even in his best races there have always been moments where Dumoulin showed weaknesses. He got consistently dropped by Yates in the giro last year, (in the purest climbing battle of the race last season, on the Zoncolan, he actually only got 5th) a year earlier he actually only finished 40 seconds in front of Nibali and got dropped by him (among others) on both of the last two mountain stages. As I said, my favorite for the race, but I don't sense the invincebility surrounding him like it does with the absolute best of the best. I absolutely think there will be stages where Nibali will be able to drop Dumoulin.
I'm not trying to argue Nibali is the favorite to win here, but I think he is up there with the other three, maybe in fourth, but a close fourth. Maybe one could argue he has a lower ceiling than the other 3 becuase he hasn't been spectacular in gt mountain stages since 2014, but then I'd argue none of the other three have a ceiling as high as 2014 Nibali (although tbf we don't actually know that about Roglic yet, I just don't expect it) and Nibali hasn't looked this promising in his last preperation race since that year. Also Nibali is most likely to go all in for the win, which might hurt his podium chances, but after all it's the win I am writing about.