Machete is the standard opening. I've played around with getting a Doran's Blade first and it is also very strong. It's mostly preference. I would still get a machete after the Doran's if you do choose to open dblade though. I start games by grabbing the ward trinket. You can use it early to ward your enemy's jungle or your own, but more important you can use it in ganks to gap close by comboing it with your safeguard! Once I get my sightstone, I switch to the sweeping lens.
Boots aren't as big of a deal anymore so finish Spirit Stone before you get your boots. It'll vastly increase your ability to clear the jungle safely and timely.
Now you can get your boots.
Sightstone is sick because it lets you almost infinitely ward hop and let's you ward important chokes/buffs. The downside is it's 475 gold exclusively for wards. There's 180 hp, yes, but a Ruby Crystal gets you that for exactly half the cost. It's still a very important item early on, but be careful about rushing this if you need to put your gold elsewhere, such as if you are extremely behind and would rather get more hp.
Ninja Tabi if you feel like you REALLY need the armor and won't benefit from the boots of mobility, but you should usually be able to use the boots of mobility to your advantage, especially if you get them this early.
At this point I would finish my Spirit of the Ancient Golem. It's a great overall item for Lee Sin and after a lot of games with many different jungle items I've decided this is my favorite over all item. It's also especially great because it allows you to get Boots of Mobility which I feel I have underrated in the past and have been building more often now.
I cannot stress how great of an item Aegis is on a melee bruiser. It gives you all the defensive stats you need for a cheap price and only costs you one item slot.
Transition to late game
Great item to start the late game with. Finish Omen's very fast if you don't need MR right away.
If you need MR, buy this right after
Hexdrinker is a great hybrid MR/AD item. On top of the MR you get from stats you also get a magic shield which is great vs burst.
If you don't need MR, finish Randuin's Omen
Finish Ruby Sightstone when you have the extra gold.
Great choice if you feel like you pose a threat to the enemy team and they want to focus you down for some reason.
Nice if you feel like you need to increase your damage. I very rarely get this item because I rely on my team to do damage but sometimes you have to be a super carry :)
My average build order
Transition to lategame:
Really strong item on solo lane Lee Sin but for jungle you simply don't have enough income to afford it.
I like Hexdrinker more but the new Spirit Visage is definitely not bad. If you need a shit ton of MR this is not a bad buy.
Really nice item that crazy increases your dmg and gives CDR. It also allows for the option to upgrade it to a Black Cleaver in the future. The problem with getting this item is that you need defensive stats on Jungle Lee Sin because you will simply not get farmed enough to be able to assassinate enemies. It's a must-buy on Solo Lane Lee but I don't recommend it on jungle.
Items that I don't buy but deserve a mention:
The problem I have with Triforce is that it takes too long to buildup. While you're building it you'll be sitting on awkward stats such as Crit, Attack Speed, Ability Power, and Mana. I definitely can't recommend this item over Frozen Mallet but if you're a Zelda fan I probably can't stop you :/
Decent item. The movement speed bonus is pretty baller and the increased ASPD helps you proc your passive more which means more energy, yay! The problem with youmuus is that it gives you only offensive stats. You'll be very squishy if you get it so I only recommend getting this in some assasin role.
I actually sometimes get this when I play top lane Lee. It's a nice nuke to combo in with all your burst. Like I said before, you need defensive stats on Jungle Lee Sin because you will not get enough gold in the jungle to be a carry.
Never buy these
You're probably surprised that I put Wits End in here. Wits End is regarded as one of
the strongest MR tank items in the game. The reason you shouldn't but it on Lee Sin is because attack speed is a wasted stat on him due to his passive. You're better off spending gold on an AD/MR item (hexdrinker!)
Optimizing for CDR is dumb on energy heroes because your cooldonws are limited by energy, not by the actual cooldown of the spell. Boots of Swiftness is just an obvious no. You're going for a very minor speed increase and missing out on tenacity/mr or armor/reduced damage. And zerkers are an obvious no because like I said earlier, attack speed is a wasted stat on Lee Sin due to his passive.
A lot of the gold that goes into this item is to pay for the CDR and Mana stats. You don't need either. You're much better off getting Omen's for armor.