KeyStone Mastery for Yasuo?

Warlord's is probably best on him. Grasp isn't super great, since he doesn't build much HP and doesn't need the early sustain (he already trades really well with his shield). I guess you could make an argument for Fervor, since he stacks it pretty easily with his Q spam and the on-hit effect on it, but I'd say it's not as strong, just because he doesn't naturally build into a lot of attack speed or use autos as his main source of damage.

Fervor is best against other melee champs where you want to win lane early. Think against champs like Riven, Fiora, etc. Warlords is generally better otherwise.