There's nothing much to say about me,although I want to say I was inspired by Faker's Lee Sin mid (I don't know if it was his idea tho)
This strat works,but it's cheesy,and cheese can be devastating but cheese can be also trash.Cheese definition in LoL : https://www.reddit.com/r/summonerschool/comments/23v0te/what_does_cheese_mean/
Hope you enjoy this guide and rate it (Even if you if you leave a dislike or w/e it's called on MOBAFIRE it will be more helpful),and comment (That really helps me)
Lee Sin'' is a mobile ad caster,while going the full AD build your damage scales really hard into the late game but teamfighting against organized teams can be a pain in the *** since you are a squishy *** assassin,focus on picks late game.
This guide is almost done (Or done?),I've worked quite hard on it,so don't be harsh :D

I've found out that this build is quite useful to shut down carries that feed off squishy mid laners,champions like Fizz,they just can't against you (if you play it correctly)
You are an anti-carry!(And a carry too obviously...)
I carried quite a few games with this cheesy strat.

+ Can shut down anyone with his absurd damage
+ Abnormal damage and assassin potential all game
+ ADCs are your food
+ High mobility
+ Good disengage with R,also you can set up ganks with it
+ E + Hydra give you enormous wave clear,which lets you be a good split pusher
+ Has access to both lanes,so he can gank them with his jungle traits
+ Tanks will hopefully fall out of meta in season 6 due to the new ADCs and ADC items
+ Energy user (Good in the earlier stages of the game)
+ After getting a Hydra+having 2-5 points in W you will never need to worry about sustain as long as jungle camps are up
+ If he loses he can transition to tanky unlike most other assassins/carries

- Energy user (Bad later in the game)
- Hard to master
- Can't teamfight nearly as good as Tank lee
- Scales in damage but falls off in utility and defense and is generally unable to teamfight due to tanks protecting the carries
- No ranged wave clear
- Extremely Q reliant

After researching a little bit I think ArPen is superior to flat AD on Lee Sin because he also has good base damages for ArPen to work with him,although if you think otherwise feel free to switch the ArPen marks for flat AD marks,that's my personal preference.

Marks: Flat ArPen goes a long way against squishies (You see that one little crit rune?That scumbag will win you a close duel once in a while,trust me),Seals : Flat health against anyone (Armor seals were nerfed,RIP armor seals ;c),Glyphs : MR against APs in the mid lane,pretty obvious),Quints:Flat AD quints for your ridiculous AD scalings and better last hitting

Cunning
Savagery - Makes last hitting easier
Assassin - Favors your assassin playstyle
Merciless - Another damage boost (when the enemy is low on HP)
Dangerous Game - My favorite mastery,this will prevent you from being killed by the enemy minions/turret after a close fight
Precision - Flat ArPen is really good
>>KEYSTONE<< - Thunderlord's Decree - Burst damage yay!This keystone really helps with your burst

Ferocity
Sorcery - For more spell damage
Double Edged Sword - 3% more damage for the penalty of taking 1.5% more damage.Worth.
Vampirism - A little bit more sustain is always good.This part of the tree is really weak considering Vampirism isn't so good at the early game and the counter part Natural Talent is really **** unless you are an hybrid champion
Bounty Hunter - Booty Hunter*.Makes you deal 5% more damage for killing is exactly what you would want.
OR Oppressor - Lee doesn't have much cc besides your E slow but it's still a somewhat decent mastery.

Dragon's Rage> Sonic Wave> Tempest> Safeguard provides the most DPS,max W second if you need more mobility and sustain.
Getting a point in W when you reach level 8 increases your sustain by a lot,not worth for one point of E

Okay so with the introduction of the new preseason Youmuu's Ghostblade was buffed and will be your core damage item. Tiamat was changed,feel free to experiment with it . Black Cleaver sounds nice with the penetration,AD ,Health and CDR it gives. Maw of Malmortius is needed against AP and oh boy it also gives flat ArmorPen now.
Serrated Dirk is the new The Brutalizer and it's really great and will be built into Youmuu's Ghostblade

Go and steal your jungler's bird camp :^) since you are a jungler naturally so it won't be hard.
Also gank for bot lane a lot.

Play Video
Credits go to Nathaniel B.

Also please land your Q's,it's important.

You can InSec your enemies to your tower ( Sonic Wave -> Stealth Ward/ Stealth Ward + Safeguard behind them -> Dragon's Rage -> Resonating Strike) and then clear the minions with E + Tiamat/ Ravenous Hydra and it will be most likely a guaranteed kill.

Your Resonating Strike is an execution ability so use it to finish your combo.
If you use Dragon's Rage on someone he will be unable to move while being in mid-air. Because of this you should try to hit your Sonic Wave right after using Dragon's Rage - you can't miss it then.

Fast harass combo :
Sonic Wave> Resonating Strike>AA> Tempest> Safeguard to a minion
Far range max damage combo :
Sonic Wave> Safeguard to a ward/minion> Dragon's Rage> Tempest>AA> Resonating Strike
Kill combo against high mobility enemies :
Ward> Safeguard> Dragon's Rage> Sonic Wave> Resonating Strike> Tempest>AA
Setting up for a jungler gank combos:
Insec :
Sonic Wave> Resonating Strike>Ward> Safeguard> Dragon's Rage

Ward> Safeguard> Dragon's Rage> Flash behind the enemy while casting> Sonic Wave> Resonating Strike
(You can Sonic Wave before starting the combo)

AD : Attack damage
AP : Ability power
AS : Attack speed
Caster : A champion who mostly relies on his ability casts to do damage
AA : Auto attack (When you right click on an enemy unit,your champion will AA)
ArPen/Magic Pen : Armor penetration and Magic penetration
Poke : Doing damage without retaliation

Patch 5.22 -
Wards were removed,upgraded trinkets are there from level 1 - Less gold spent on wards,clearly a buff for Lee
Tiamat change - Way cheaper but passive removed ;c
Black Cleaver change - Costs more,gives less HP but gives more AD now necessary since Last Whisper is bad ATM
Trinity Force - Less attack speed but more crit chance and 10% CDR.Always an item to consider
Youmuu's Ghostblade - Crit is gone for 35 more AD,this item is really good now

Lee Sin is a very fun and strong champion,and can be played in various roles (Jungle,Mid,Top hell I've even seen him support).
You are free to change the build to your liking and this is only a base to the final build of each and each game played in the mid lane with Lee Sin.
Hope you liked it,be sure to rate and comment,I've worked quite hard on this guide,thanks in advance.

For penta in the game oh boy,sorry for not being able to post the image on here it just bugs out.
http://imgur.com/S9jKjbv
More results:
http://imgur.com/xV0qIWD
http://imgur.com/E3HbHwc

To me and to MOBAFire for this site and Nethaniel B. for the video I provided.
Hope you learned something new :D

24/4/2015 - Guide created
01/5/2015 - Changed name from Patch 5.7 to 5.8 and added relevant info from the patch to the new chapter "Relevant Patch Changes"
14/7/2015 - Guide slightly updated with important things to note about patch 5.13
15/7/2015 - More updates
1/11/2015 - Bringing this guide to life with a complete revamp (Glossary added,Pros/Cons,items,masteries and runes were updated and more.
13/11/2015 - With the introduction preseason 6,the items and masteries were updated and the off-tank build was removed
20/11/2015 - Combo sections reworked and masteries changed
11/3/2016 - Changed masteries and changed the wording a little bit