Well, to go all mythological on you, Fëanor’s hubrus was his greatest sin. It’s not just that he was selfish, but that he was so very proud of himself and all his works so that he really perceived himself to be better than everyone. While I wouldn’t say he thought he was better than Eru, I do think that he really did put himself above all the other elves and probably also the Valar themselves. Going to the Teleri and telling them “your ships are mine now” basically, and then betraying his half-brother and other kindred…and then trying to attack Morgoth and thinking that he actually could defeat a Valar? Delusions of Grandeur.
Fëanor is an interesting character, but I can’t stand the bastard. I realize that Fingolfin was far too classy to do this, but I always imagine that when he reached Mandos, Fingolfin found Fëanor and sucker punched him. I kind of imagine a line of Elves, kind of like in the movie Airplane, lining up to smack Fëanor around.
As for the OP…I think the kinslaying is the worst deed of Fëanor although you could nominate Ar-Pharazôn for a lesser prize in wanting to invade Aman and take over the undying lands.