Thunderlords is generally the best keystone available for Lee Sin. It gives great damage on the third hit/ability and can save your life in duels/team fights
Most of the mastery points chosen are self explanatory, but I'll go over some of the more difficult decisions.
Runic Affinity, Secret Stash, Assassin: Between all 3 of the given masteries, I prefer Runic Affinity the most. The 15% longer buff duration is fantastic for clearing through the jungle. Red Buff, Krugs, and Gromp will all increase your damage drastically while clearing while buffs like Raptors, Blue Buff, and Wolves will help you scout, sustain, and protect your jungle. The other options are decent, but not nearly as good as Runic Affinity. Starting with Secret Stash, while the biscuits are really nice for healing while jungling, it's simply not needed as you can already sustain quite well Lee's W and Refillable Potion. As for Assassin, Lee Sin is great in team fights and you should be looking for plays with your team. Because of this, the Assassin mastery isn't worth the 2% extra damage while fighting separated from your team.
Feast, Double Edged Sword, Expose Weakness: First off, let me just that out of all the options, you will pretty much never use Expose Weakness as the other masteries are just simply better. Because of this, there is a difficult choice between either Feast or Double Edged Sword. I personally prefer Feast from the two options. The extra damage is really nice on Lee Sin, especially because you will probably be dueling with the enemy jungler quite a bit. However, I do not like the extra damage taken in return. I'm sure many of you believe dealing extra damage is absolutely worth taking some extra damage in the process. I on the other hand love the small yet important healing from Feast. I know I keep bringing it up, both having a healthy early clear is so important for Lee as you'll probably end up power farming for (Level 6).
Bounty Hunter, Oppressor: This one isn't as complicated as the others as both options work just fine, but I want to explain whats good about both. Bounty Hunter gives 1% increased damage for each unique enemy champion killed which is great for Lee being very strong early, assuming you get 3 unique kills. On the other hand, Oppressor is great with nearly all champions considering most have a form of CC (Crowd Control) such as a slow, stun, root, taunt, etc. This works nicely with Lee's slow, along with your teams CC to back it up. I generally take Oppressor because I find it more reliable, but you'll do just fine with either.