Hi this is one of my first build.
So hope you like it!
I've played LoL for soon to be three years now and I've played with several champions.
But Lee Sin is so outstading if played right that he's my favorite champion.
This build works for me and represents what i think is best to Lee Sin. So i would appreciate any feedback given... :)

I go with:
6x Greater Mark of Strength
3x Greater Mark of Desolation
9x Greater Seal of Resilience
9x Greater Glyph of Shielding
3x Greater Quintessence of Strength

Which gives you:
12.45 Attack Damage
4.98 Armor Penetration
12.69 Armor
1.35 MR per lvl(24.3 MR at lvl 18)

Here is a Jungling guide for Lee Sin by Stonewall08 Play Video
I prefer to jungle Lee sin that way Stonewall08 show here starting wraiths>red. If the enemy mid push the lane you can level up Q and make an early gank.
Or else you can level up W and continue with wolves>blue then gank.
Even if Lee sin doesn't have mana he still can take the first blue for CDR or else give it away.

Lee sin can also steal baron easily by sending in an Q. If you hit you'll get vision of enemy team plus barons health. Then you can wait until the baron gets low on life and set your wriggle ward down W and smite baron. Or you can do it with setting down the ward then wait for the baron to get low lifed. But in higher levels the enemy team often buy an oracle and clears out the wards.

Here is another guide from Stonewall08 for Lee sin in Teamfights. Play Video

I prefer:

Why Smite? Because you want to be with as much health as possible when you gank.
You clear the jungle much faster so you can gank more often or counter-jungle.

Why flash? You can still catch up your enemy when failing your Q.
Plus you get an extra escape.

Viable options:

Exhaust is a viable option because its easier to kill. You lose less health from the enemy.
It helps you land your Q. And double slow :D

Ghost is a viable option but I dont use it because Lee sin already have a gapcloser skill to hunt enemies down with. And you escape more easily with flash.

Pros

-Energy Based (doesn't need blue that much)
-He's a strong duelist
-He's got CC
-Gapcloser
-Shield and Lifesteal
-He can choose his own path (both in jungle and in items.)

Cons

-Very little sustain before Frozen mallet.
-Prone to getting nerfed.
-Need to hit with his skill kit to gank.

The conclusion is:
He has very little cons.
He can choose his own paths.
Has very high sustain after Frozen mallet.
Can iniate an teamfight easily and steal barons.
He's got pretty good damage early game.
He clears jungle camps very fast.
Easy to counter-jungle with.