Starting items:
-Dorans Blade/Dorans shield
-Health pot
-Wards

Critical Strike:
-Phantom Dancer/ Static shiv :Either of these two items work well on Yasuo but ultimately it comes down to preference of item. Personally i prefer pd for the higher as and the unique passive but if you are versing a team of mainly squishies static shiv is a solid choice but if their team consists more of tanks or bruisers pd would be the way to go.
-Infinity Edge : A viable option but i normally only buy it after frozen mallet if i am ahead in the game and if i can afford it.

Core:
Frozen Mallet: Yasuo's q procs this items slow effect which allows for you to kite them while also dashing around so the slow makes up for the delay on your q and allows you to create some distance and be very obnoxious. This items synergies with the phage passive because you will be slowing your energy and moving faster than your enemy. This is why you buy phage right after your pd/shiv then build triforce after frozen mallet.
Triforce:
-Phage: the movement speed for kiting plus the base ad and hp stats
-Stinger: CDR means windwall and ult will be up even faster but the 50% increased as is very nice to get that 1.33 sec cd on q
-Sheen: Additional CDR and the sheen proc give your q more dmg, your e aa more dmg and helps siege towers faster.

Defensive items:
AGAINST AN AP HEAVY TEAM:
-Banshees
-Spirit Visage
-QSS
-Maw of malmortius

AGAISNT HEAVY AD TEAM:
Dead mans plate
Randuins omen
Sunfire cape
thornmail

OTHER DEFENSIVE CHOICES:
-steraks gage
-titanic hydra
-GA
-Warmogs armour