Offensive:
Double-Edged Sword : Can secure a kill in a 1v1 fight. Altough when you're alone vs 3 people you will take more damage.
Fury : gives you some bonus attack speed, makes your Q cooldown lower.
Brute Force , Martial Mastery and Warlord : Some extra AD
Spell Weaving : basic attacks gives your abilities more damage, good with Yasuos low cooldowns.
Executioner : gives you a better chance to execute an enemy.
Blade Weaving : combined with Spell Weaving gives some extra dammage.
Frenzy : gives Yasuo more attackspeed, this is beacause of his double Crit chance.
Devastating Strikes : some basic armor penetration.
Havoc : 3% extra damage, what do I have to say about that?

Defensive
Block and Unyielding : Less dammage on Yasuo.
Recovery : Yasuo has no sustain, this way he gets a little bit more health regen.
Veteran Scars and Juggernaut : more health is more time to kill enemies.

Way of the Wanderer: Very strong double passive.
The first passive is Double Crit chance, this way you have to invest less money in critical strike chance. The con with this passive is that your critical strike damage is lowered by 10%.
The second passive is your flow. Yasuo has no mana but he got flow. Flow is not used for casting spells. You build your flow up by walking around, the faster you walk the more flow Yasuo gets. Once attacked by an enemie champion or a monster the flow will act as a shield. This makes Yasuo good against other ADC because it will counteract the first basic attack early game.

Steel Tempest: is your bread 'n butter, each succesfull strike gives Yasuo one stack. At three stacks Steel Tempest will send out a tornado which sends enemies airborne. The attack apply on hit effects.

Sweeping Blade: gives you your mobility and makes you charge your flow faster. It has a very low cooldown but after dashing on a enemy champion or minion it marks him, making it undashable for Yasuo until the mark disappears.

Wind Wall: is your main ranged ADC counter ability, they won't be able to hit you when you stand behind your wall, this way you can push them out of the lane. The higher you level your wall the lower the cooldown will get and the wall will get wider. When maxed at level 18 Yasuo is almost able to close up a whole lane. It blocks all the basic attacks and almost all skillshots. It counters Miss Fortunes and Ezreals ult, making it a good life saver.

Last Breath: The ult of Yasuo requires some skill to use in the right way. Enemies need to be airborne before you can use Last Breath. Your Steel Tempest knocks enemies up, but only after two succesfull hits you will be able to cast the knock-up. This requires some skill. You will be able to use knockups of allies like Vi, Malphite, Wukong and many more. Some chamions knockup themself like Ziggs with his Satchel Charge making it very easy for you to cast your ulti. Last Breath has a good range , and all airborne enemies are knocked up another extra second with Last Breath they all take the same damage and yasuo gets bonus armor penetration (50% for 15 seconds) making it rewarding casting Last Breath early in a team fight. On Last Breaths max level the cooldown is only 30 seconds making it possible to cast it two times in a long team fight.

Steel Tempest: Max this one first, it is Yasuos main damage output.
Sweeping Blade: gives you your mobility Max this second
Wind Wall: I pick one point in Wind wall early so I can 'dodge' enemy ranged attacks.
Last Breath: Take this whenever possible, each rank the cooldown of Yasuos ult gets lower. End game there is only 30 seconds left of the cooldown.

You can pick Sweeping Blade on level 2 instead of Wind Wall but I prefer Wind Wall early so I can counteract enemy ranged damage. In combination with your shield from Way of the Wanderer you get verry less damage early game while you push the enemy ADC away from her farm.

Last Breath: Take this whenever possible, each rank the cooldown of Yasuos ult gets lower. End game there is only 30 seconds left of the cooldown.

starting items: Doran's Blade gives Yasuo the early game durability. With one Health Potion he will be able to stay long in the lane early game. Stealth Ward for free warding early game.

first back: Boots to chase enemies better and charge your flow faster. A Zeal for some attack speed and crit chance. The zeal only gives 10% crit but it is doubles with Yasuo's passive giving him 20% crit chance early game, which means you crit every 5 attacks. Steel Tempest is able to critical strike and its cooldown is lowered with attack speed.

Core items: Statikk Shiv has great stats and a perfect unique passive. The static charges build up with movement just like Yasuos flow. Steel Tempest apply on hit effects which gives Yasuo a good wave clearing. Berserker's Greaves for attack speed and lower cooldown on Steel Tempest. Vampiric Scepter gives Yasuo the sustain he needs because he has no build in sustain. The final core item is Infinity Edge because of the double crit chance. Infinity edge and statikk shiv gives Yasuo 90% critical strike chance.

Ravenous Hydra, Bloodthirster and Blade of the Ruined King against enemies with very much HP I recommend Blade of the Ruined King. With alot of enemy squishy champions a Ravenous Hydra would be perfect because it will gives you splash damage. Most times I go for Bloodthirster, it is a balanced item and gives you alot of burst damage. This way you are able to easily assassinate backline enemies.

Defense options: Most times I already won the game bevore picking defensive items. Randuin's Omen gives Yasuo health and armor, its passive makes 1v1 easier. Because Yasuos able to chase enemies by dashing through minions its passive makes it able to slow a running enemy champion. So your team can pick up the kill.
Warmog's Armor gives you alot more HP, because Yasuo does not got that much of HP it could be a good option if you get focused. Guardian Angel gives Yasuo another life to kill enemies, it will make the enemy team upset, when you're finally dead you get another chance. It gives some armor and magic resistance too which is nice. When you're full builded and got nothing to spent your money on, you can sell Guardian Angel and buy it back to reset the passive timer.

Boots replace ment Phantom Dancer and Zephyr gives a bigger bonus then Berserker's Greaves and Statikk Shiv But only do this when you are full builded.

Roaming: when you can pick up some kill in other lanes, or they are having a hard time, buy Mobility Boots instead of Berserker's Greaves. Making it able to swtich lanes very fast.