Lee Sin is a great champion in-lane.
The Assassin build is so that you go AD Carry with a Support. In there, the main aspect his Trinity Force and Bloodthirster. If you can have thoose 2 items, you have a greater chance to get kills. ( Trinity Force his for all it's buffs, do not wonder why i choose since Lee Sin doesn't use MANA).
The main aspect of the Tank build is Atmog's ( Atma's Impaler + Warmog's Armor), but there are some aspect to maintain Atma's Impaler at his best, like Force of Nature. In the Off-Tank build i choose Madred's Bloodrazor because of his passive: "Your basic attacks deal bonus magic damage equal to 4% of the target's maximum health". Then, we also have the assassin build that aims you to get the most advantage of lee sin's abilities for a greater damage.

Greater Seal of Armor and Greater Glyph of Magic Resist are just so that you can old your lane better against a Ranged Champion (Attack Damage or Ability Power).

In my assassin build, i choose to put 2 Greater Quintessence of Desolation to get extra armor penetration and the
Greater Quintessence of Health to get the durability in late game.

The choice of Greater Mark of Desolation and Greater Quintessence of Desolation is because Lee Sin above all his an assassin, so armor penetration his normally a good choice to get kills.
I also choose Greater Quintessence of Health to get some extra health for the Atmog's and for some more durability in the beginning.

(You can use the other rune build if you prefer cause you what you lose in armor penetration in the other, you gain in health.)

Assassin Build

I believe that thoose 21 points into Offense give Lee Sin a great damage output through all the game.
The 9 points in Utility are so that you can get to your lane faster, to help you with any buff or if you spend you're energy too much on harassing your enemy champion.

Tank Build

I believe that when going tank, thoose 21 points into Defense will help you stay in your lane for much longer and it helps you in the case of a gank.
The 9 points in Utility are so that you can get to your lane faster, to help you with any buff or if you spend you're energy too much on harassing your enemy champion.

Pros:
+ Very good farmer
+ Good at harassing from early game
+ Probably the best at getting first blood in the game
+ You can knock champions out of a channel (Nunu, Malzahar, Warwick)

Cons:
- Have to be able to use the Sonic Wave / Resonating Strike
- Takes time to learn what to prioritize in situations like teamfights and 1v1.
- Needs time to learn how to play (like most champions)
- Hard not to use W on yourself all the time, instead of saving it for getting out of tough situations (especially when chased)
- Need to control you abilities to take the most advantage from Flurry

Flurry (Passive)
After Lee Sin uses an ability, his next 2 basic attacks gain 50% Attack Speed and return 15 Energy each.


Sonic Wave / Resonating Strike
Sonic Wave: Lee Sin projects a discordant wave of sound to locate his enemies, dealing 50/80/110/140/170 (+ ) physical damage to the first enemy it encounters. If Sonic Wave hits, Lee Sin can cast Resonating Strike for the next 3 seconds.

Resonating Strike: Lee Sin dashes to the enemy hit by Sonic Wave, dealing 50/80/110/140/170 (+ ) physical damage plus 10% of their missing Health (Max: 400 damage vs. Monsters).


Safeguard / Iron Will
Safeguard: Lee Sin rushes towards a target ally, shielding them both for 40/80/120/160/200 (+0.8) damage over the next 5 seconds. If a shield is broken, half the Energy cost is returned. After using Safeguard, Lee Sin can cast Iron Will for the next 3 seconds.

Iron Will: Lee Sin's intense training allows him to thrive in battle. For 5 seconds, Lee Sin gains 5/10/15/20/25% Lifesteal, Spell Vamp, and 10/15/20/25/30 Armor.


Tempest / Cripple
Tempest: Lee Sin smashes the ground, sending out a shockwave that deals 60/95/130/165/200 (+ ) magic damage and reveals enemy units hit for 4 seconds. If Tempest hits an enemy, Lee Sin can cast Cripple for the next 3 seconds.

Cripple: Lee Sin cripples nearby enemies revealed by Tempest for 4 seconds, reducing their Movement and Attack Speed by 30/37.5/45/52.5/60%. Movement and Attack Speed recover gradually over the duration.


Dragon's Rage (Ultimate)
Lee Sin performs a powerful roundhouse kick launching an enemy champion back, dealing 200/400/600 (+ ) physical damage to the target and any enemies they collide with. Enemies the target collides with are knocked into the air for a short duration.

Assassin Build

In my assassin build i get a Doran's Blade and aim for Mercury's Treads right away. Next i try to get Trinity Force for excellent damage after the use of an ability and movement speed and right away i try to get Bloodthirster for the damage and the lifesteal. Next is Frozen Mallet because, as in all assassins, a slowed enemy gives you a great advantage to get a kill. Now the the fourth item depends on the enemy team. If they have a tank you should get the
Madred's Bloodrazor instead of the Frozen Mallet. If they do not have tanks, you should get a Frozen Mallet first. Then a Youmuu's Ghostblade for attack damage and more armor penetration. The next item (Regular Build) his Infinity Edge so that you have a great damage and some critical strikes.

Tank Build

In my tank build, i get a Doran's Shield and aim for Boots. If you are playing against a caster choose Null-Magic Mantle. The boots that are best for Lee Sin are Mercury's Treads because of Tenacity (reduces the duration of stuns, slows, taunts, fears, silences, blinds and immobilizes). Then come the Atmog's. Always start to get the
Warmog's Armor and next the Atma's Impaler. The next item depends on the enemy team: if they do not have tanks, go for Force of Nature; if have a tank, take a Madred's Bloodrazor. Frozen Mallet it's just for the it's passive that helps you ganking champions that are trying to push. To end the tank and only tank build i choose Sunfire Aegis because of it's passive that gives you a kind the burn of the Lizard Buff, and combined with the Frozen Mallet, it's like having a Lizard Buff all the time. It also helps to get more damage from the Atmog's.

I believe Flash and Ignite work very well with Lee Sin, since the Ignite let's you finish off the enemy champions faster and the Flash let's you get close to them easier, or if they try to escape.

You can also use Exhaust instead of Ignite if you prefer to try and get kills from your enemies faster.

Ghost is also a good choice if you think that your Lee Sin doesn't not need Flash. Ghost also let's you get to your enemies faster or if your enemies are pushing or you need to help a teammate, Ghost may let you get to him faster or get to the turrent where they are pushing faster.

I hope this build help you get a great score with an assassin/tank Lee Sin that can easily go off-tank.
Vote, comment and say what i need to improve.