Xin Zhao is a completely dominating carry if fed, and with the right build and a little skill he is unstoppable in team fights. He can put out insane damage, and if he is not focused, he can destroy entire teams.

I play Xin as a solo mid carry, or a jungler. While this build is designed for these roles (if you want to jungle check out my jungle build), you can try this build on a side lane with varying results. I find that to successfully play Xin in a side lane, you have to get fed, something you cannot count on in every game.

This is my first build, so go easy on the comments. I have not tried this build in ranked games, because I am not level 30 yet (level 29). Please try the build a couple times(some people judge a build without even trying them in game) to leave a constructive comment before down voting, so I can make changes. Also, remember that this build won't win you every game. You have to adapt the build to counter the enemy team.

Pros:

OP if fed
Excellent damage output
Great solo mid
Excellent in teamfights
Great mid game
Excellent ganker

Cons:

Squishy early game
Vulnerable to CC
No escape

I run ArP marks and ArP quints because they are extremely good buys on a physical DPS carry, like Xin. I go with AS seals and glyphs because I don't build too much attack speed in my items. However, you can go with anything you want for seals and glyphs. Greater Glyph of Scaling Cooldown Reduction, Greater Glyph of Scaling Magic Resist and Greater Glyph of Magic Resist are all good choices on Xin. Greater Seal of Defense, Greater Seal of Evasion, and Greater Seal of Armor are also good choices. In addition, you can go with Greater Quintessence of Health to give you additional survivability early game.

I work all the way down the offense tree, because the extra damage is helpful early game, where Xin is weaker. You can choose to put your 9 remaining points in either the defense of utility tree. I find it helpful to get the extra xp bonus and the buff duration masteries because Xin is OP when he is a couple levels ahead of your enemies and has red buff (and blue if my team doesn't need it).

Core Items:
Berserker's Greaves (or Mercury's Treads depending on enemy)
Wriggle's Lantern
Frozen Mallet

Situational:
Warmog's Armor + Atma's Impaler: if enemy has high physical damage output
Force of Nature: if enemy has high magical damage output
Phantom Dancer: a good item on Xin Zhao but not particularly important
Last Whisper: another good item, but if you build ArP marks and quints you should have enough armor pen
Banshee's Veil: a good item on any champion
Black Cleaver: a great substitute for the Bloodthirster, because it is easy to apply all three stacks in a relatively short duration.
Quicksilver Sash: basically Cleanse, but better (it has saved me many times), a great buy to counter champions with annoying ultimates (like Malzahar and Mordekaiser)
Infinity Edge: a nice 6th item that will greatly increase your damage output late game, a very good buy if you are not getting focused in fights.

Elixers are great items to purchase when you don't have enough money to buy any item in your build. On Xin, the most effective elixers are Elixir of Fortitude and Elixir of Agility. However, if I have to pick between the two, I always go with Elixir of Fortitude. The extra health increases your damage (because of atma's) and it also gives additional AD.

Excellent:
Exhaust: The key to Xin is keeping your opponent in place. With Exhaust, you should be able to land a full combo twice. With Exhaust, you are pretty much guaranteed a kill when it is off CD.
Ignite: Ignite is a great finishing skill early game and is perfect for throwing that extra damage on to finish off enemies. It is not that useful late game, but is great for disabling champions like Mundo, Swain, and Vlad, because it reduces health regen.
Ghost: A lot of people choose Ghost over Exhaust. It is very viable, but it just doesn't fit in with my play style. I would recommend getting the Improved Ghost mastery (instead of the reduced death timer).

Good:
Heal: Heal is decent on a champion like Xin. Because he is so squishy early game, it can help keep him in lane longer and farm a bit more. Late game it can also be useful to keep your team alive in teamfights. However, there are better spells that can be used on Xin.
Flash: Xin doesn't have an escape mechanic. Flash is useful to get away from ganks and it can be used to get that extra distance to land your E. However, like Heal, there are better spells that can be used with Xin.

Xin is a great solo mid against squishy ranged casters (like Karthus). His high damage output will destroy enemy casters in mid. However, he is not a good choice against champions like Pantheon in mid. In the past, I have had trouble farming against good Pantheons.

People Xin CANNOT Mid Against
Vladimir
Pantheon
* This list will be updated in the future, because I have not tried mid against everyone

Xin is viable in a lane with somebody with high CC or extremely high damage output. He is excellent with champions like Brand and Blitzcrank because of their stuns/pulls. I have also dominated in lane with champions like Garen because his excellent early game will help you farm up and even get a couple kills.

I start of the game with Boots and 3x Health Potion or a Dagger and a Health Potion. It is important to always grab Health Pots because until Xin gets Wriggle's he is very squishy. I try to farm as much as I can until level 3, when I have all my abilities. Your Q is perfect for farming. The extra damage makes it easy to last hit and the relatively low mana cost and CD make it spam able. Xin needs the farm early game so that he can rush Wriggle's. The lifesteal, AD, and armor are so important early game, because it allows you to stay in lane longer. In fact, it is so important, that I sometimes get Wriggle's before I finish boots. If you are mid, try to let the lane push back until level 3, so you can get a kill on the enemy. The goal of going mid is to get kills, out level, and out farm your enemy, so that you can ultimately dominate late game. There are games when I have gone mid that I end up a couple levels higher than everyone else, because I constantly force the opponent to base. If you are in lane, at level 3, coordinate an attack the squishier of the two enemies. Let your lane partner CC the enemy, then E in, Q, and once you FINISH knocking them up, W. Throw on Exhaust, and if you have not already killed them, use Ignite. If I get the kill, I usually base and finish my boots, as well as start my Wriggle's Lantern.

Once I hit level 6, I usually have a couple kills and a decent farm. I base the moment I hit six and finish my Wriggle's and look for a lane to gank. Xin is one of the best gankers in the game, because of his AOE slow and hard hitting ult. Try to target a squishy lane (I usually gank mid if I am not mid). You should be able to net a kill, or a double kill if your lucky. After the gank, base again and grab as much of Frozon Mallet as you can (making sure to grab Giant's Belt before Phage because the extra health is invaluble).

Mid Game is where Xin dominates. Once you get your Frozon Mallet you are unstoppable. The best advice I can give is that since Xin has no escape skill, pick your fights wisely. Too many times I see Xins run into fights that are unwinable and end up giving the enemy a free kill. Also, NEVER INITIATE as Xin Zhao. Always wait for a teammate to start a fight, then dash to the enemy carries and wreck havoc. Try to save your ult until you can hit at least 3 people because you gain additional MR and Armor depending on the number of champions you hit. Your teammates should let you get the kills, because if you get fed, you are unstoppable.

If you played the game right, it should be over by now, or will be soon. Usually, I finish the game before I can finish my build. Remember that after Frozon Mallet the build is entirely situational. If their are many casters, get FON, if they have an annoying physical DPS then get Warmog's + Atma's (Atmog's as I like to call it). If you got fed, you should be able to 1v2 or 1v3 their team. At this point, if I have enough gold, I often sell my Wriggle's Lantern for a Bloodthirster, because it gives better AD, and Lifesteal. While the active ward on Wriggle's is nice, it is not very useful late game when everybody is grouped up and you know where they are, for the most part. At early levels, Xin is a terrible turret killer, however, late game, he can destroy them with ease. At this point, the enemy is probably whining about how your so OP and you need to be nerfed for the billionth time.

If your enemy keeps running away before you can finish the knock up on your Q, activate your Q hit a couple minions and then dash in. This will result in an automatic knock up. While you will not deal as much damage, it is a great way to harass your enemies.

With Xin it is easy to pull off a double attack. Attack something, then quickly activate your Q. This is very helpful in the jungle as well as for farming.

Feel free to use your Crescent Sweep to clear creep waves, if you are sure you will not be needing it in the next 30 seconds. While the CD is around a minute, everytime you activate your W and hit a couple minions, the CD drops. It should be on CD for a relatively short time, so don't hesitate to use it.

Xin can solo dragon from level 6 - 7 onward. Whenever it is safe, try to take out dragon. The global XP and gold are invaluable. Many people underestimate the power of killing dragon.