Pros / Cons
Good sustained damage
Vulnerable to CC
Focused in teamfights
Squishy at early game
Requires a lot of teamplay to maximize his damage and utility
Sonic Wave / Resonating Strike:
Sonic Wave: Lee Sin projects a discordant wave of sound to locate his enemies, dealing 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 physical damage plus 10% of their missing health.
Safeguard / Iron Will
Safeguard: Lee Sin rushes towards a target ally, shielding them both from damage. 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 lifesteal, spell vamp, and armor.
Tempest / Cripple
Tempest: Lee Sin smashes the ground sending out a shockwave that deals magic damage and reveals enemy units hit. If Tempest hits an enemy, Lee Sin can cast cripple for the next 3 seconds.
Cripple: Lee Sin cripples nearby enemies revealed by Tempest, reducing their Movement and Attack Speed for 4 seconds. Movement and Attack Speed recover gradually over the duration.
Dragon's Rage (Ultimate)
Lee Sin performs a powerful roundhouse kick launching his target back, dealing 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.
After Lee Sin uses an ability, his next 2 basic attacks gain 50% Attack Speed and return 10 energy each.
I Chose Ehxaust because it helps a lot while ganking lanes and chasing enemies. Also, it will help you in combination with your W to mitigate a lot of damage during a teamfight.
I Chose Ghost as a secondary Summoner Spell because it helps to chase enemies and avoid unpleasent ganks.
Other Good Choices:
Cleanse will help you with CCs. Lee Sin is very vulnerable to CCs and it works great in combination of Mercury Treads.
21-8-1: I use the common 21 points in offense to maximize the damage output. Then 1 point in Improve Ghost and the rest in Defense. Getting the 8 points in defense could be a difference at early game.
21-0-9: I use the common 21 points in offense for jungling, taking 1 point in Improve Smite. Then the rest in utility to maximize the Exp gained and buff duration.
I start by maxing Q because its an awesome harras with a high damage output at early. Then W because it helps to control the lane and keep you and your friend alive and prevent you from backing with the life steal.
I assign 1 point to E at lvl 3 because the cripple, its useful to chase enemies at low lvl and finish them with Q > Q.
I start by maxing E because the AOE dmg in order to farm faster. Then W to stay more at the jungle and only 1 point in Q to gank and chase enemies.
I hope you enjoy my guide. Any constructive critisism will be appreciated.