I watched the Tour every year when they competed. Evans in his prime was not among the best climbers, but not far off either. He was quite good but not great. Especially good on short climbs or climbs which did not make differences, where he could use his sprinting ability.
But he suffered in the high mountains where he was between 3rd and 6th strongest when in top form, depending on competition.
But he was the strongest TTist of all GC contenders and (Annecy 2009 aside) not much worse than Contador and always Top 3 or Top 5 in the TTs.
In 2007 he was clearly weaker than Contador , Leipheimer and Rasmussen. In 2010 he was not as strong as Basso. In 2011 he faced a tired and crash-hurt Contador and profited from tactic games in the first 2-3 mountain stages. Nevertheless he was very good and 1 day even the strongest in a mountain stage. But I think the waiting games hurt Andy. It was strange to see them all play defensively when they should attack Evans from the first mountain finish.
But in the end, Evans pulled it off, had the form of his life and made use of the circumstances.
Only a 2011 Evans in a Leblanc or 2012 TdF would or could beat a Giro '13 or TdF 2014 Nibali. In a Giro he would never win against a top form Nibali. Too many MTF and too little TT km.
He could have beaten a 2013 Vuelta Nibali, but this poll is about their respective best form.
This year he would have lost on the cobbles, maybe stayed with him in PDBF, and lost 30-50 sec in the longer ascents. Then gained back 40 seconds in the TT. Assuming Evans was in 2011 form.
I consider Evans on par with the best Sastre in a GT. Slightly weaker in climbing but stronger in the TTs. Apart from 2008 where Sastre had extra power in yellow and Evans was slightly behind his best form. 2011 Evans vs 2008 TdF Sastre is a very tough choice. Maybe Evans wins by 20 seconds.