I just tried some games with lee sin and I think he does not have such a huge dmg output so I think the best way to build him would be as a full ad character.

I use armor penetration runes because I think his dmg is a bit low (basic attacks) and that attack speed runes would not be very helpfull, thx to his passive he already has quite a good attack speed. As Lee sin is dependent of his skill you should go cdr runes as well and to finnish I would say some health runes cause Lee sin can get quite squishy on early games.

As I said in the introduction Lee sin does not have huge dmg output with his basic attacks so I think he should be played as a full ad assasin. You could take a Wriggle's Lantern on a 5v5 map, could be quite usefull. I start with a doran's blade to maximize his dmg output at early game to make some killz otherwise he would not be able to equip correctly and I swear that Lee sin is really bad when not well equiped. Trinity force is a very interesting item for him because he uses a lot of his abilities. Youmuu's Ghostblade is usefull for chassing people and kick them back in your team with your ultimate. Lee sin needs lifesteal to stay up long enough to show his true dmg so I decide to go with a Bloodthirster. So that Lee sin isn't always killed so quickly in team fights by aoes I go with a Banshee's Veil.

Here I go 21-0-9 with Lee sin because I think again to max his dmg as much as possible. 9 point could also be put in defense but I think that Lee sin does well with buffs so I like to give him more about that + he depends a lot on his lvls so I go and take these 4 point in awareness. In my attack spec I take these 15% magic p������©n������©tratino for some of his skillz, I thing 15% magic pen is more interesting than 2 more dmg output.

I tend to maximize my "q" abilitie because I think this is the spell Lee sin does the most dmg with; his "e" abilitie does not have crazy dmg output so I leave it rank 1 to spot ennemys and slow them. His "w" is interesting as I said Lee sin needs lifesteal and this helps him alot to survivde when you add it to his shield.

I use exhaust and ghost with Lee sin like with most of my ad characters. You can play him with a flash instead of ghost but I think he has enough dashes and ghost + Youmuu's Ghostblade is good for catching up ennemys and stick on their butt, or even kick them back into your team to get an easy kill.