Hello, I'm Lord Barchy and this is my 2° guide for Mobafire.
LeBlanc is (in my opinion) one of the best Nukers on the whole game.
This is a build based on my experience, because LeBlanc is one of my main champions.
I think I'm the best LeBlanc actually in the game (at the moment I haven't seen a better LeBlanc than me), so I think this is the best LeBlanc's build.

IMPORTANTE: For this build YOU MUST GO MID!

As most of mages, LeBlanc burns mana as a car burns diesel. Because of that, we must have a nice Mana Regen stats.
For early game we will usually drawn first blood and we'll need this mana to stay on the lane and gain some LvL advantage.

Of course that you may kill the Blue Golem as soon as posible, epic Mana Regen and 2x% CD Reduction isn't bad.

You should focus your runes on Mana Regen, CD (Cooldown) Reduction and Magic Penetration.

I focused the masteried in 3 important points;
-Mana Regen
-Cooldown Reduction
-Movement Speed

Juts for masteried you will have one nice Mana Regen and something about 9% CD Reduction (it may be really easy to reach the 40% max CD Reduction without buying CD Reduction items more than Mejai's Soulstealer)

LeBlanc is a mage, and a pretty squishy one. You must not go on face melee or ranged enemys if they are full healt or if you dont have your combo.

You may start with Doran's Ring. 100HP, 15AP and 5Mana Regen extra at LvL 1 arevery useful.
With everything (Masteried, runes and Dorans Ring) you should have 24 Mana Regen at lvl 1.

As LeBlanc is one squishy champ, you must focus on scaping/killing. Buying the Boots of Swiftness will give you the advantage you need. With the masteries help you may have 412 Movement Speed more than enough to chase your enemies and smack them down.

You must focus on kills (even if you need to steal some of them) to stack as soon as possible your Mejai's Soulstealer.

After that, you MUST buy the Rabadon's Deathcap. By now on, you are ready to nuke out enemys in 2.x seconds and earn easy gold.
You MAY NOT but healt/AP items like Rylai's (unless you really need it), we are mages, not tanks, we will focus on kill they enemy nuker/killer as soon as possible in teamfigth, then help our team chase and kill the survivant enemys.

It's very useful to buy after the Rabadons Deathcap one Hextech Revolver, so you will heal % of you Magic Damage (Magic Damage that will not be insignificant).

Follow the orden given top for the items, when you complete the 6 slots, sell your Dorans Ring and buy another Rabadons Deathcape.

IMPORTANT: You must have you Mejai's Soulstealer stacked full (20 stacks) and kill the Blue Golem as soon as possible (40% CD Reduction).

PS: The first item on the list is the Doran's Ring, I just deleted it because I mistake >.<

You must focus on lvling up Glyph of Silence, hit your enemy multiple times with this spell and try to low his life as much as possible. At lvl 2 you must lvl up the jump (Distortion) and earn first blood as soon as possible. The most of the times you will earn it as you lvl up to lvl 2. If you did everything well, your enemy may be 1/2 HP. Then you must use Glyph of Silence + Distortion + Ignite + 1 Normal Atack, with this your enemy must die, exept if you it maybe difficult to lane against some tanks like "Alistair".

When you reach lvl 6, you will have your full nuker combom wich is the following:

Q+R+W+E (Sigil Of Silence + Mimic: Sigil of Silence + Distortion + Ethereal Chains).

Most of times your enemy will die with the first 3 spells, if your enemy doesn't, you may have to ignite him and snare him with Ethereal Chains, as the time comes and he's sanred you should be able to use your Sigil of Silence again, finishing your enemy.

LeBlanc has 3 "standar" combos you can use:
-Nuker: This combo is mean to kill your enemy in less than 3secs. It's Q+R+W and if the enemy is scaping you can use you E + Ignite.

-Scape: Its very easy to use. You just have to click W + R in the direction/way you want to scape. If the enemy is geting close you you, you can use you E to snare him and take some time to scape.

-Support: This one has 2 usses;
-Kill Support: This is very useful when you are atacking the enemy tank. You have to snare him with E and after that snare him again with R. Then you can apply the basic combo (Q + W) and he may die.

-Team Support: This is overwesomly useful when you must help one of you teammates. If the enemy is chasing you teamate you can kill them with the Nuker Combo or you can apply this support combo. You must walk carefuly infront of your ally and then use E, snarin one of you enemys after that, you should wait one second and then use R to snare another enemy again. If you apply it well, you may have saved one of you teammates :)

This spells are special for miding. As you bother your enemy with you Q Spell and low his healt you will reach lvl 2.

As soon as you fhave you W, use the primal combo (Q + W) and kill him with ignite.
Before you jump, you must quick click on your health and then Jump (Distortion) + Ignite, hit him with standar atacks 2 times and then go away. Congratulations, you may earned first blood.

Reach easy 800 AP with Rabadons Deathcapes and your Mejai Stacks. Have one healing point on your Spell Vamp thanks of the Hextech Revolver (and later the Hextech Gunblade). Knock out your enemys in less than 3 seconds and farm them as you ahve never seen.

As you saw on my build, I dont buy HP items. Why? Because we're nukers! Not tanks, leave HP to Singed or Alistair, but you must focus on your AP.

If you followed everithing correctly, you should be able to kill alone enemys with 3000-4000 HP alone and without ding, and the best part; all that in less than 3 secs!!

Heres is one image that proves that his build pwns, it was a normal game 5v5:
My best run with LeBlanc was 44/2/21 in a 5v5 normal game at lvl 30.
Hope the guide serves you, any doubt you can post it free.

This guide was made completely by me, based on my game experience with this champ.

Enjoy!