Angliru 2013 is the race I would point to if I had to explain why I like Nibali as a gc rider so much. What I hate more than anything is riders with only a small chance of winning behaving like they have no chance at all.
Nibali clearly wasn't climbing as well as Horner that race and was already trailing him by a few seconds. He didn't expect he could just ride him off his wheel so he attacks at a spot where Horner didn't expect it yet and would actually have to close a gap. He closes the gap anyway, Nibali is already on his limit, f#ck that he attacks again. Horner is closing the gap again, it's at this point 99.9% clear he can't drop Horner that day, f#ck that even more, what about the 0.1%, let's accelerate again.
Seriously, I don't think I have ever seen a rider so desperate to drop someone. Don't think there is a better example of a rider who you can visually see goes so much deeper than he should actually be able to. That climb was pure fighting spirit by Nibali.
It would be easy to point at someone like Bardet or Uran and say "see, they could have tried more going for a gt win" but I honestly feel you could point to pretty much everyone except Contador and do the same. Froome in particular. As great as he was, and although he ended up getting his big gt comeback win in 2018 there are two examples in particular where I felt like he just didn't ride solely for the win. In 2014 when he made one huge attack and then just rode his pace on the mtf's Contador ended up winning, he couldn't possibly have thought that was the best way to gain enough time, right? And then two years later it was very similar when he started to attack Quintana on like the last 3 or 4 km of stage 20 or something. He really needed to gain a big chunk of time yet attacked so late that he likely wouldn't have gained enough, even if Quintana had had a bad day.
That's where Nibali is usually very different. Even 3 years later when he got 2nd in the Vuelta again with another Angliru stage on day 20, he again knew that he couldn't win that race by just outclimbing Froome, so he attacked twice on a wet descent, in one case leading to a crash that basically closed the chapter "Vuelta win". But anyway, he attacked at the point where he had to attack if he wanted any chance left to win the whole thing and that's just something I really admire.