Many people have tried multiple builds on Xin Zhao, since he is a very versatile champion and any item fits him well, but I think that with this build xin zhao is best played, a very well known Xin Zhao player, Guardsman Bob, uses this build for Xin Zhao himself.

The runes I go for are pretty basic on any melee, armor pen reds, quints, armor yellows and mr/lvl blues.

You can swap the yellows with dodge runes if you want.

I go for 15/0/15, it gives me enough masteries for the extra AD, and also improved Flash, so I think it's really good.

Other options would be:
21/0/9 (getting extra time on neutral buffs)
0/9/21 (Summoner Spell CD is really good and Nimbleness is very useful, especially if you go dodge rune yellows instead of armor)

These items I consider the best on him, Trinity gives all Xin needs, and Atmas gives him good amount of tankiness and damage.

Some people go for ghostblade instead of Trinity Force, but I think Trinity is better because Ghostblade, apart from being an Early-Mid game item, when you use the active, you are pretty much done, Trinity Force gives you that Movement Speed buff forever, it's really nice on Xin Zhao.

Boots Ninja Tabi or Mercury Threads. NOTE that if you are going for ninja tabi you should includesomewhere in your build.

-- ITEMS TO AVOID --
- The passive from this item helps, but it isn't worth the gold, don't waste item on this.
- Same as Black Cleaver, costs way too much, and gives you no type of defense, sure, you will have a lot of dmg, but you can't use it because you will die before dealing it, also AD doesn't scale that well on Xin.
- Don't get these boots on xin, or any melee, as a melee you will be in the fight, so you will go better with defensive boots.
- Xin zhao pretty much focus down one target after his initial Ultimate burst, so the passive from the cape won't be very useful.

People have discussed many times about what to Max first, Q or E, I am kinda sure the damage is pretty much the same if you go one or the other, but the thing is leveling E increases the slow % and lowers the cooldown, so It's better to level it up faster.

Flash and Ignite are the best, but that's just personal choice.

You can also swap Flash with Ghost and Ignite with Exhaust.