This is my first time ever setting up a guide/build, but I will try to add more and edit this as patches come along and if I find a better/more builds for him. Lee sin is amazing and i agree with many others when I see he is prime with an Atmogs, so I decided, if hes best with an Atmogs, why not get more defensive items and just build mostly tanky. Well here it is, hope ya like it.
The runes are mostly to help out his early game with survivability, while still giving him a little bit harder of a hit with the armor pen Marks and 1 Quintessence. The health quints, armor seals, and magic resist glyphs will help him stay in lane a bit longer so he can at least take a few extra hits.
masteries are opposite of the runes, which balances him out so he can still do some decent damage even late game with all/mostly tank items. 21/0/9 is what i see to be more common for him, but then people go straight ad/as, which isn't always bad, but by the time you've got a good combo going, especially late game you've already taken too much damage and will end up dying, so that's why the runes are the way that they are, as well as the items in game.
I find him best built tanky so that way hes not always focused in a team fight, but can still deal out some decent damage. I find the item sequence i have to be best, but obviously all games are different and you may change an item or two, or even get some items before others; every game is different (I usually stick with the same build but usually change the last item depending on the other teams composition, if theyre more heavy AD than AP ill get a thornmail instead of Force of Nature), so some situational items would be: Thornmail, banshees veil, Force of Nature, Spirit Visage, or even another Warmogs. A thornmail would be best agianst a team that is hitting harder with their AD champs than their magic users. Banshees veil is really up to the person(you already have hexdrinker, so if you plan on getting this just replace hexdrinker with youmuus Ghostblade), i dont like buying it bc the catalyst is only partially useful to Lee Sin, and you waste 400 gold on a Sapphire Crystal(but its up to who you're against, especially if theres a karthus, you dont want to have to leave base after fully healed to have to return just because he casted his ult and took a decent chunk of your health). Force of nature I find best if youre going against a decent team with an AP hard hitter/carry, plus that's even more helath regen to keep you in game without having to go back to the well. Spirit Visage isnt really all that good to get, but its cheap and itll give you 30 magic resistance, 250 health (which also adds more ad due to the Atma's), and the extra 10% cdr if you really want it. and last but not least another Warmog's Armor, if you find yourself not taking all that much damage you could get yourself one of these and add another 1,370 more health and 28 more AD making you hit even harder and able to take a lot more damage. Youmuus ghostblade is something you can get instead of hexdrinker if you plan on grabbing banshees veil, its nice bc youll get extra armor pen and 15% cdr, along with the active which is nice because of lees ability to chase or escape easily.
Your skill importance goes, Q, W, E, and leveling your ult every chance you get at 6, 11, and 16. Your Q is your hardest hitter and gets lower of a cooldown every time you level it, Your W shields you and gives you spell vamp, life steal, and armor if you hit it a second time. Youre E is mainly just to help you chase because it reveals units for 4 seconds after the initial first hit, and hitting it a second time will slow the movement/attack speed of anyone hit with it the first time. Youre Ultimate is amazing, you can use it to run away, defend a fellow team mate by kicking the enemy away or THROUGH terrain, or get someone out of a team fight thats causing too much cc or damage so your team can gain the upper hand, it also cancels channeling abilities such as Nunu's Ult or cause fiddle to cancel his ult before its fully channeled.
Flash is great defensively and offensively, it will help keep you cahsing someone if theyre just out of range of your q, or you can flash behind a wall/away from an enemy and that could give you enough range to use your q on a neautfal monster camp, minion, or even use your w shield to jump to a fellow ally through another wall or further down a lane. Lee sin is very good when it comes to chasing or running away, he slows, knocks back and has two leaps depending on surrounding minions or allies/enemies.
I definitiely stole some of the items, and all of the runes, masteries, and skill sequence from EvilDice's Guide, but the rest (including the SITUATIONAL items)is my view and opinions On the build and champion himself. I will add more as the times come for patches that affect him, and if i find better builds whether they are tanky or built more towards hurting, but I dont see him as a good burster, I see him best built tanky/offtank.