I'd have to go with nick, the guy's run in strikeforce was insane. Not only who he beat but how he beat them. Marius Zaromskis, Paul Daley both he refused to take them down and work submissions. He just walked forward and threw strikes until he won. Not many fighters can do that without being kfto. Also he fought larger fighters and even stepped up to face Anderson silva. Madness. He even took that nasty lead right hand jab-thing that fucked everyone up at 205 lbs... motherfucker ate it like it was nothing at all. Weed PEDS give you an iron chin and cat-like reflexes yes, but still.
Nick was in a completely different category than other fighters in my opinion. He wasn't the best in his weight class really ever, but the way he fought i always admired the way he fought. You could tell that all he did was get high and box all day long and a fight was no different. He didn't care who he fought or what their style was, it was just go in and throw volume punches until his opponent falls. Even when he got stung badly, like in the gomi fight, he just found a way to win.
Nate was I would say slightly more professional, but he didn't have the dynamic amazing moments like nick had in his career. Yes he had the McGregor fight, and the double middle finger triangle, but I just feel like nick stood out more and was more capable at different weight classes and in different settings.
I suppose I'm going based off an overall summation or their work, and being that it is quite literally art, it is hard to judge or say anyone is wrong. The numbers in careers rarely speak the truth.