I love playing Yi, and in s6 he's stronger than ever. People build him with Devourer/Rageblade/Blade of the ruined king and to me that's lost potential. For a rundown of his abilities in depth, go to the wiki. This guide will be about:
1. Building a AD/CRIT/TANKY Master Yi
and
2. Why you shouldn't build devourer on Yi.
I will comment on other Yi guides on Mobafire and I encourage you to read them. I won't cover Master Yi's abilities, Jungle Paths, Warding Spots, Dragon Timings. Hope this guide helps you and drop a comment if you have a question

<Part 1 Ferocity 21p> First is attackspeed, because the more attacks the more times your passive is up and you can double strike. A point in Feast gives 20hp whenever a unit is killed which is good for jungle sustain. 5p in Vampirism gives 2% lifesteal, also for jungle sustain and is amazing lategame(spellvamp isn't useful). 1p in opressor, slowed targets take 2,5 extra damage, which goes well with chilling smite (1p Bounty hunter is an option, can give you up to 5% extra damage total). 5p in Battering blows for 7% armor penetration (maybe the best 5p you'll spend for master yi) and 1p in Fervor of battle, which gives extra damage depending on how many times you hit a champion. Warlords bloodlust is an option, it gives 30% attack speed and heal when you crit a champion. Really tough call. Sustain is good if you're up against a Dr. Mundo. However most of the times you will blow up Varus and Viktor in 2seconds so it's not that important

<Part 2 Cunning> 5p in Savagery for faster jungle clears. 5p in Wanderer gives 3% mov speed when out of combat. This is great lategame, but early on clearing faster will let you gank more. 1p in Runic Affinity gives you 15% longer buffs. You can go 1p in Assassin, which gives 2% increased damage if you are without allies. I like longer buffs, because 15% is significant, and Red Buff on Yi is very strong.

<Part 3 Resolve> 5p in Unyielding gives 5% extra Armor and Magic Resist. Much better than passive health regen which is the other option. 1p in tough skin means you take 2 damage less from monsters and champions. Great for junglers

I don't go deeper into Cunning or Resolve because a mix is safer for Yi, and he is fragile and squishy early on. 5p Merciless gives you 5% increased damage vs champions below 40% health, but if they are so low, you've already won probably. Bandit and Dangerous Game are ok, nothing special. Meditation is good on yi for the Mana, but I like the tankyness more from Resolve-tree.