Skolas? Beat him on my 2nd wipe.
Crota? Ah pretty hard, but I still beat him.
Oryx? I beat him after returning and leaving three times because of frustration.
Atheon? It was hellish
Phogoth near the release of Destiny. One of the bosses you had to be overleveled for to not get completely wrecked. And I'm talking about the basic default on you select on the planet!
Oh and ahhh I remember the good o' days when Phogoth was a nightfall/weekly and it was so difficult. I was in love with Suros Regime and my friends bought into the hype and got it from Xur. The nightfall went from impossible to semi-hard real fast with the Suros Regime.
Honestly, my group hits every new raid blind. No info. Warpriest probably took us the longest because we didn't realize the shadows kept us alive. Lol.