Hi and welcome to Lee Sin, The Shojin Monk.
In this guide, i mainly focus in health and in some lifesteal, so that when in battle you have some advantage then just counting in your health. I will explain why i believe that theese are some of the best features that Lee Sin should have. So that all that health does not go to waste, i add a Atma's Impaler so that your health turns into damage. I believe that when going wiht lee sin, you should solo top. Jungling is also a good option.

Laning

I choose to put 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.

Jungling

When jungling, i think that lifesteal and the extra damage against monsters that the Offense masteries give is the best option.
The 9 points in Defense are so that you can have your own Thornmail to jungle and so that when walking to another camp, you get your health at max faster.

Laning

Greater Seal of Armor and Greater Glyph of Magic Resist are just so that you can old your lane better if you are being harrassed a lot early game

The choice of Greater Mark of Desolation and Greater Quintessence of Desolation is because Lee Sin his an assassin, so armor penetration his normally a good choice to help you 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.

Jungling

Greater Seal of Armor so that you take less damage from mnsters and Greater Glyph of Magic Resist so that you can stay alive when helping in a lane against an Ability Power champion.

The choice of Greater Mark of Desolation is because Lee Sin his an assassin, so armor penetration his normally a good choice to help you get kills.

Greater Quintessence of Attack Damage so that you can jungle faster and take more damage when ganking.

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, etc) with Dragon's Rage

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

Laning

Normally, i start with 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, in my opinion, Mercury's Treads because of Tenacity (reduces the duration of stuns, slows, taunts, fears, silences, blinds and immobilizes). Next, i get the Wriggle's Lantern so that you have some lifesteal to help you stay in your lane and so that you can farm faster because of extra damage. Wriggle's Lantern also gives you the ability to do ward jumps. Next, i start to build an Atmog's, and i start for the Warmog's Armor and when you gonna get the Atma's Impaler, your Warmog's Armor passive should be full. Next, for more damage i get a Trinity Force. My last item is the Frozen Mallet for more health to get more damage from the Atma's Impaler, some extra damage that it gives you and for it's wonderfull passive.

Jungling

When i go jungle, i start with a Vampiric Scepter that with your Safeguard / Iron Will gives you some great lifesteal for early game jungling. I always aim for Wriggle's Lantern and next for Mercury's Treads. Next, i start to build an Atmog's, and i start with the Warmog's Armor and when you gonna get the Atma's Impaler. Next, for more durability i get a Force of Nature so that if you get to very low health and recalling is a bad option, you can stay where you are untill your backup comes. My last item is the Frozen Mallet for more health to get more damage from the Atma's Impaler, some extra damage that it gives you and for it's wonderfull passive.

Optional Items

Guardian Angel if you are having problems staying alive because you are targeted, this item will help since it gives you Armor, Magic Resist and a great passive
Thornmail if the enemy team as a lot of Attack damage champions, this will help you stay a live and maybe kill one of them in a 1v1
Quicksilver Sash if the enemy team have a lot of CC effects, the active of this item will help you
Randuin's Omen has a good passive and a great active that may save you and some of the champions on your team
Trinity Force is a good item when you wanna go with a Lee Sin that deals more damage since if gives you some great buffs

When i jugle with Lee Sin, i try to take an advantage from it's passive. I start with Safeguard / Iron Will because the shield and the lifesteal will help you more.
Normally i start at Golems, do the Wraiths and then the Wolfs. Next i go Golems Again, Lizard Camp, Wraiths, Wolfs and Ancient Golem Camp. Before you go Golems for the second time, see if your team needs a gank in any lane and be always watching.

Laning

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.

Jungling

I take Smite when jungling for obvious reasons.

I choose Ghost because when jungling, if a member of your team dies and he his going solo, his turrent is alone and the enemy team get's a free push. And when jungling there is no need for Flash because of Lee Sin's ability to ward jump.

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