This guide is strictly for advanced players, with a little tweak to Critorius' build, Insane Mid. I tried his build, but I thought something was missing. After numerous tests, I find this build to be superior to any other build currently listed. If you don't believe me, you should try it out and see for yourself! DO NOT DOWN VOTE !!
Please read the whole guide before you vote. Thank you.

Everything is standard except that one quintessence is spell pen. This is because we want to maximize damage as much possible. Since most champs have 30 MR at start, having Sorcerer's shoes would make their MR to 0. This is very crucial to LeBlanc's burst.

There is no doubt that flash and ignite are the best spells for LeBlanc.
You will realize you don't need exhaust or teleport like Critorius' build because when it comes to team fights or ganks, you are either going to be one shotting someone or busy escaping. In addition, your distortion and mimic are probably down because you just used them bursting someone. So, flash is a must for LB, like insurance. With ignite, you make sure you get a kill. Ignite is a must to kill tanks or heavy lifesteal melees.

This is the rotation you use to burst down any champ.
If he isn't dead, you use Chains ability to make sure he doesn't get away.

Early Game:
Never waste spells on minions. He has a very low mana pool.
Keep harassing champ withand. You will easily get an early kill if you harass them with this combo with ignite.
Mid-late Game:
Never waste spells on minions unless you have blue buff.

After countless games with LB, I can say that this build is the best.
Please try it and let me know what you think. Thank you for reading.
KOREAN STYLE BABY.