Who am I?
I'm a casual player that has been playing since season 4 and I've been maining Xin Zhao for three years. I hardly ever play ranked Because of the toxicity problem but when I do Xin is sure win. His kit contains hard CC with his third q, a slow with his w, a gap closer with his e and invulnerability with his ultimate and not only that but he has extra damage, an attack speed buff and a built in heal. So he is a perfect champion for new players to have fun and for veterans to hard carry their teams.

Xin Zhao is a really flexible champion and can be viable with every rune path
You can pick:
~From the Precision tree:
1) Press the Attack
2) Conqueror
3) Lethal Tempo: more of troll rune but still it can work out especially in late game
4) Fleet Footwork: this rune gives a good amount of sustain but there are better options
from this tree and you can get Hp from Hunter's Potion and Blade of
the Ruined King
~From the Domination tree:
1) Hail of Blades: great synergy with his q
2) Electrocute: viable when you get a lethality build
3) Predator: It is a rune that can work when the enemy team has a lot of escape options
and you have to chase them down
4) Dark Harvest: when all enemies are squishy and you want to get the lethality build
~From the Sorcery tree:
1) Arcane Comet: this rune is more fitted for lanes to poke people with you w
2) Phase Rush: Great rune to counter enemy's kite and to give you the little oomf to keep
up with them
~From the Resolve tree:
1) Grasp of the Undying: this rune goes great with full tank Xin Zhao
2) Aftershock: You can proc this rune with your third q or with your ult. So you can dive
in the enemy team pop you ult and have a ton of extra resistances for
every one that wants to come close
3) Guardian: good rune to protect your team especially when you have a lot of melees
~From the Inspiration tree:
1) Unsealed Spellbook
2) Glacial Augmentation: this rune is perfect for champions that Xin Zhao can't quite
reach with his limited mobility
3) Kleptomancy: more suited for lanes you can proc this with your q very easily
Now for the smaller runes:
~Precision:
Triumph is op and every champion that goes through the precision tree should take it except
for mages that take Fleet Footwork. Next take Alacrity or Tenacity depending on the
situation. Lastly Coop de Grace gives the most damage out of the three, cut down if the
enemy team comp has three or more tanks and last stand isn't the best rune but it will give
you more damage the closer to dying you are. (In a situation where you and an enemy is low
the best rune to have is Coop de Grace because it deals more damage)
~Domination:
Sudden Impact is you go to rune choice since it gives you a good burst of lethality, Cheap
Shot will make your third q and r do more damage and taste of blood will give you a bit
more life steal but its better to build life steal than pick this rune. (Overall Sudden
Impact since lethality in the early levels is really good because no one has built armor
yet). Next Zombie Ward, Ghost Poro and Eyeball Collection. Zombie Ward has a good synergy
if you get the oracle lens and use it before ganging. Ghost Poro is great to use when you
are counter ganging and need the vision in the enemy jungle and as a bonus it gives you a
small amount of attack damage. Eyeball Collection is a rune that you cant go wrong with,
you kill ward and champions and as a reward you get extra damage. Lastly the Hunter
Quartet. Ravenous gives you a good amount of life steal, Ingenious is a must pick if you
took the Predator rune, Relentless gives you a good amount of movement speed to shorten you
time from one gang to the next or to reach an ally who needs you help, and Ultimate is a
bit of an overkill since you lower you abilities cooldown when you attack with your q.
~Sorcery:
From the first trio only Nimbus Cloak and Nullifiyng Orb are worth. Next Transcendence is
a good rune to pick for the extra CDR and the extra stats you get when you exceed the CDR
cap. Absolute focus isn't the best choice here since in fights you rarely will have more
than 70% Hp. Celerity After the change is great for champions that have a lot of movement
speed but it will not give you anymore a good movement speed buff so, for Xin, its great
when paired with waterwalking so you get a good buff when you fight for objectives in the
river such as Scuttle Crab, Dragons and Baron. Lastly Scorch works best for poking
champions during laning phase, waterwalking gives you great mobility when you are in the
river and is a must pick rune when its paired with Celerity and Gathering Storm is a rune
that will help more and more as the game drags on
~Resolve:
Demolish is a great rune if you build HP and you want to split push, Font of Life isn't the
best choice for Xin but you can heal your allies with your third q and your w and bone
plating works best during laning phase to block some damage. Next Conditioning is the
recommended rune for the extra resistances it gives after the 10 minute mark. Second wind
will give you some more sustain after the gang. Chrysalis will give survaivability in the
early game and damage at the mid-late game. Lastly Overgrowth will increase your health
pool quite a bit especially when you jungle so its a good rune to pick and it becomes even
better if you build HP Since it increases your HP by a percentage of your max HP every 8
minions/monsters that die near you. Revitalize makes your passive heal you more and
Unflinching will give tenacity when you use a summoner's spell (e.g. Smite).
~Inspiration:
Hextech Flashtraption will make some of your gangs unpredictable, Magical Footwear is
basically free gold at 10 minutes or earlier but you might need boots earlier and if you
don't get kills or assist you will be slow until you get the boots and Perfect Timing is a
good rune if you plan on buying Guardian Angel or Gargoyle Stoneplate. Future Market isn't
the best rune because when you use it you will have 50 gold minus but its only worth it if
you want a little bit more gold to complete a full item. Minion Dematerialize is useless in
the jungle. Biscuit Delivery heals a lot in the late game and it increases your mana pool
by 200 if you use all of them but they occupy an inventory space and you can heal by using
Hunters Potion. Lastly Cosmic Insight gives you 5% CDR on your abilities, items, summoner's
spells and it increases your CDR cap by 5%. Approach Velocity will help you reach allies in need quicker but the champions you impair are already in your attack range so this rune isn't the most effective. And lastly Time Warp Tonic Will extend your potion duration by 20% and will also give you a small movement speed buff.

Skirmisher's Sabre is a must buy on Xin because you have a gap closer with your e and it also maximizes you dueling potential. Trinity Force is a must buy item for Xin since he need all the stats from it and he use them to stomp all over his enemies, however this is a very expensive item so you should buy this only when you are farming with ease and no one is counter jungling you and you have a good economy in general. T he standard build order for Xin is first Phage for the damage, health and movement speed it gives you, next Stinger because he utilizes the attack speed and CDR really good and if Phage didn't gave so much HP and AD Stinger would be the first component you should buy and lastly buy the Sheen because Xin doesn't need more mana and the extra damage isn't the best considering the cooldown it has so you can't use it more than once or twice during a combo. Next Steraks Gage gives you a lot of HP, a good amount of AD and you get a shield when people try to burst you down. That was the core of the build after that the next items are entirely situational.
For defense Randuin's Omen gives you a lot of armor and a good slow, Spirit Visage is a go to item when you need MR and as a bonus you heal more with you passive, Thornmali will also apply Grievious Wounds to enemies but this should not be the first armor item you buy because the passive where you deal damage to enemies that hit you is almost non-existant if you don't have any more armor. Also Warmog's Armor is a great item to pick against a heavy poke team.
For offence you will need a Blade of the Ruined King since it's stats, the attack speed, lifesteal and damage, are really good for Xin and for his all in potential. Titanic Hydra is good for when you have a lot of defenses and need some damage. Guardian angel give a good amount of damage and its passive is really good if you make a mistake during the team fight and you die but be careful because it has a 5 minute cooldown.
For boots you should pick Mercury's Treads over Ninja Tabi only when the enemy team has a lot of cc and/or more than three AP champions.
For utility always have a Control Ward to place it the dragon's pit, baron's pit and a good place to put it is at the bush behind the red buff but place it in a way that looks close to the raptors and the path that goes to mid and to the river because a lot of junglers pass through there without checking their bush so you can see where he is and intercept him, counter gang the lane he is at, counter jungle or simply to warn you teammates. Also the Elixir of Iron is a good choice to get when you are the one getting focused and/or need to tank and peel for your team, and the Elixir of Wraith should be picked when you are the one who needs do all the damage in the teamfights.

Xin Zhao is a very flexible champion and according to the match up and the situation you can max any ability you need. In short, you level up your R whenever you can and after that you level your Q for more on hit damage and to reduce its cooldown, W for more damage (This ability deals the most damage to champions) and to reduce its cooldown. E for more damage (although this ability scales with AP) bigger slow and reduced cooldown.
Generally maxing W first maximizes Xin's DPS, maxing E gives him more utility with the slow, the attack speed and the gap closer and Q provides only more on-hit damge
Also the best combos you can use is E>Q>Q>Q>W with this combo you have your attack speed to make your opponent airborn as soon as possible and then hit him with your W and since he is airborn and can't move you can't miss it.

Pros
+Great duelist
+Great and healthy jungle clear
+Good gangs with your E
+Huge carry potential after only two items
Cons
-Easily kitable
-No AoE skills to use in teamfights
-Without his R he can be focused and killed easily

In summary Xin Zhao can have many roles that he can play in a team comp, but the most important is to know how to adjust your play style in different situations. However these thing don't really matter if you are not having fun, so the most important thing to do when playing Xin Zhao is to enjoy the games. Hope I helped you understand this wonderful champion and good luck with your ranked games