In my opinion, Leblanc is a strong champion who excels at catching the enemy off-guard and make seemingly impossible plays due to her massive mobility and damage. Her playstyle is definitely high risk-high reward.

In this guide, I will share my experiences as LeBlanc with you, dear reader.
I will first go through her setup and general playstyle, and then go into specific matchups and when to pick her.http://
I will not go into most general concepts such as smartcasting, ward placement, baron traps, ect. There are plenty of guides out there explaining that already, links to those things will be included in the bottom of this guide.

Before this guide takes off for real, I have two things to say:

1. LeBlanc requires more awareness and faster reflexes than most champions in order to master her completely. If you can't pull combos and jukes off fast enough, your potential skill level is greatly crippled, and it is probably a good idea to play another champion.
By extension, it is also crucial to master smartcasting if you wish to become good with LB, and your Ping should be low.

2. The contents of this guide applies both to toplane, midlane and support LeBlanc, as all lanes are viable

Choice of runes


Marks:
9 Greater Mark of Magic Penetration.

TL;DR: No alternatives to Magic Pen. It is simply too strong.

Nostalgia alert: R.I.P. Hybrid Pen runes patch 4.20. You were much loved.


Seals:
Greater Seal of Scaling Health
Seals are made to be defensive, and although some extra damage from AP or scaling AP seals might be tempting, it is simply not gold efficient enough compared to the extra HP you can get. Best choice of seals if you fight AP or hybrid enemies. Also, the scaling Health will make you more tanky against burst damage lategame than any other seals, including flat or scaling armor.

Alternatives:
Greater Seal of Scaling Armor
Armor runes can be good against heavy AD teams and AD lanes, which you might face if you play LB top or into the likes of Zed or Talon mid. However, after the nerfs to flat armor seals, there isn't really a good option for early game power against AD lanes anymore.
Usually, scaling armor seals only make you more durable against burst damage when compared to scaling health when the enemy team has more than ~80% Physical damage, so they should only be taken for the added sustain in lane, NOT because you wish to be less squishy lategame.

Glyphs:
Greater Glyph of Magic Penetration
Probably the most controversial choice in my rune page, Penetration glyphs actually outclass scaling AP at ALL stages of the game except for waveclearing. More math incoming at the end of this chapter.

Alternatives:
Not really anyone, you usually lose out on too much damage if you don't take penetration glyphs. On the other hand, maybe that's just my agressive playstyle talking.
Greater Glyph of Magic Resist could be useful in some lanes where damage is abundant or undodgeable, such as against Annie or Lissandra, but that should not happen often. Look at the matchups section to see where they might be viable.
If you don't have Magic Pen Glyphs, get them ASAP. Period.


Quintessences:
Greater Quintessence of Ability Power OR Greater Quintessence of Magic Penetration
As I stated earlier, the choice of quints is all about early/late tradeoff. Magic penetration generally beats Ability Power if you want to deal damage, and the same goes for the Quints, but with one exception. Flat AP Quints gives you more damage levels 1-7 and about the same damage levels 8-9 compared to Magic Pen Quints.
Magic penetration "scales" with both your AP and level (ability base damage), so it will end up adding ~75 damage to your combo late game, while you lose out on 8 damage with your average level 2 combo and 6.5 damage with your average level 3 combo. Also, your level 6 all in will deal about 16 damage less if you opt for Magic pen quints instead of AP quints. The reason you lose so much damage level 6 is because of the low base damage and ridiculous AP scaling on your ult, which makes AP even better early on.
Since you probably combo the enemy a lot early on, all of this damage adds up in lane, and the prospect of landing a killing blow where you might otherwise fail leads me to recommend Flat AP quints because of the importance of early game.
If you instead prefer going for late game dominance, you can opt for Magic Pen Quints because of the ridiculously strong scaling they have later on in the game, which gets really noticeable starting at level 11.

PS:
Since early kill pressure in lanes got reduced in the 4.20 patch that buffed everyone's health, Magic Pen Quints are looking more tempting than ever for a more "late" focus. Go with whatever you please, just remember the breakpoint is around level 8.

More detailed explanation in the lovely math section at the end of this chapter.

I personally use the following rune page for LeBlanc:

Boom Boom baby LB: (Yes, check it up on op.gg. That's the page name.)
9x Greater Mark of Magic Penetration
9x Greater Seal of Scaling Health
9x Greater Glyph of Magic Penetration
2x Greater Quintessence of Ability Power
1x Greater Quintessence of Magic Penetration