Nibali is underrated.
The reason IMO is that he didn't improve enormously from one season to the next, but improved steadily until he arrived where he is now.
Turned pro young, and was regarded as a huge talent. First Giro as nr 3 or 4 at Liquigas, did well enough, but compared to A. Schleck of course nothing. Improved in 08, 09. And after 09 he was still regarded as clearly inferior to his teammate Kreuziger. Winner of the TdS 08, but in 09 behind him in the TdF, and until Nibali crashed in the stage to Prato Nevoso in 08 (a crash which clearly weakened him in the days to come, plus he had already done the Giro) he was ahead of Kreuziger too. Before the 2010 season Nibali was actually at least as strong as Kreuziger, at least. IMO slightly stronger. Generally though it was clear, Kreuziger a possible future great, Nibali, ah well, not so bad.... he lacked the flashy win.
Then breakout year in 10, 3rd of the Giro, being the second strongest. While helping the strongest, Basso. Winner of the Vuelta.
2011 clearly a set back, 3rd in the Giro again, but this time weaker than Scarponi and a bad Vuelta. After the Giro 10 you'd have expected Nibali to be clearly better than Scarponi.
2012 the Tour again, got third, the only one who even tried attacking the Sky monster team, before the Tour he wasn't the no 3 favorite, more like 5-6, behind riders like Gesink or even Van den Broeck. Dominated those guy, while having no chance against Wiggins and Froome.
13 we were told he had no chance in hell vs Wiggins at the Giro.. .he won. Then beaten by Horner in the Vuelta.
14 he won the Tour easily after Froome and Contador were out.
So steady improvement at least until 13, not sure if he improved further between 13+14... but somehow that doesn't register with some people, they like to see him as the same guy who won the Vuelta in 2010, he is way stronger than he was then now, or the one who had his only "non improvement season" in 2011.
Similar situation IMO with Uran. Pro very young, started of well actually, but didn't immediately win a big race. Stagnated, then finally found his GT legs in 2011, pretty good TdF, got weaker at the end. Next year the Giro 7th, and white jersey (even though Henao actually looked stronger overall), 2013 second in the Giro, light years from Nibali, but still second. 2014 second to Quintana... and here... pay attention in the downhill to the Stelvio and he might actually have beaten Quintana. So Uran 2014 was pretty damn close to Quintana, still, Quintana will be in everybodies top 4 GT riders, Uran way back. Quintana at this point is better, but Uran is not that far off. But.. like Nibali he didn't simply explode one year and then stay there, but improves a bit every year. Which seems to throw people off. Uran IMO will be a harder opponent for Contador at the Giro than Aru will be... but suspect most will see it differently. Because Aru exploded in 14 (to be behind Uran nota bene) while Uran 'just' improved.
Same as with Nibali. Now... where does Nibali stand compared to Froome and Contador? Hard, or impossible to say, since in 13+14 there was just one kind of meaningful direct fight... Tirreno 13 vs Froome (Contador had his off year, so not meaningful with regards to him). Froome won the mountain, Nibali the crazy rain stage... in the end not too many conclusions from that one either. Indirect comparisons, pretty meaningless. With Valverde and mountain-TT losses, probably in 2013-2014 was similar in the end. Which doesn't say much because the race was different. What I think? Guess? Froome in the TT is clearly better. While I suspect that by now Nibali is actually slightly better in TT than Contador (Nibali started off as a good TTer, then somehow seemed to get worse, back to pretty good TT from 13 on) Mountains: Froome nr 1.. but he did have worrying weaknesses in 2013 at the end, both Nibali and Contador don't usually. Here probably Froome-Contador-Nibali, but all very close, all within 20" on mountain top arrivals. Take into account some Froome weaknesses, lack of consistency and it's Contador-Nibali-Froome... But all just guess work.. That's why they actually race... and: Riders that don't explode from one year to another but develop slower aren't necessarily worse than the exploders when all is said and done.