Hello,
my name is Master Yísus and I am at the moment a low diamond player on EUW, who only plays Yi.
I have played this build in quite a lot of different patches and it always worked, although the ardent, shields meta was a pure pain.
Play it, get good at it, learn how to get ahead early and you will see results.
I am not saying this is the perfect build, or that I am the best player, but I had success and fun with it.
I tried a few builds and imo this is still the best build in 8.13, with some minor tweaks, you can see above.

!!Don't forget to open the notes to see my thoughts about items, runes and so on!!

Atm i am back to using dark harvest, because i feel like i am good with it in the early game and more squishies are picked in general.
Normally i build a bf sword and then a zeal, depending on how much money i have.
I always finish statiks then, since IE rush isn´t an option anymore.
After my 100% crit i go bloodthirster and then it all depends on the enemy, as already explained above.
The crit build is not that strong anymore vs squishies, but better vs tanks and since 8.13 guinsoos got worse too, so crit is still perfect to play.

Also i always ban jax or warwick and I buy pots more often now, cause of the fact, i feel like early fights and invades, aswell as scuttle fights are increasing.

Better:
- Take out squishies faster, nearly instant
- Needs no time to build up ( no stacking guinsoos)
- First auto attack is nearly always guaranteed and deals the most dmg
- Solo carry teamfights by insant killing squishies and q your way to victory
- early powerspike very strong
- even harder snowballing potential


Worse:
- shredding tanks takes way longer
- objectives take longer early
- less surviveability, since you can´t / it´s way harder to fit def items into the build
- Dark Harvest takes time to build up, strong impact at 150+ stacks
- when behind pretty weak early and mid

Pros
- insane damage
- solo carry potential
- very high snowballing effect
- high cdr
- fun to play
- very rewarding

Cons
- insanely squishy
- reliant on getting ahead early or stalling
the game
- high risk
- hard to play

There are basically 3 routes to start the game off. 2 for the bottom side, 1 for the top side.
For the top side you want to go:
blue -> gromp -> wolves -> chickens -> red -> krugs -> top gank
When you get a good leash you want to smite the red and the krugs and use your meditate after wolves and before doing red.
When you get a bad leash smite the group.

For the bottom side you want to do red, then invade the enemy wolves. Only do it if you think the enemy didn't ward the river. After that do chickens and then go directly to your blue, because many players will be frustrated and try to invade your blue.

The second route is just a full clear, but you skip krugs. After you did your gromp, get scuttle and gank top or mid if possible. Often you can counter gank top.

8.11 till now: Now it is worth to go for scuttle after blue and gromp. In General you want to focus more on scuttle, but still think about if you can fight their jungler or if it is better to go a safe route.

I don't think I need to explain how to gank with Yi. You can lane gank, come from behind or do whatever fits the situation.

If teamfights happen, you want to wait for someone to engage. Either the enemies or your tank. When the players burned their skills you want to flash on the squishiest and most dangerous target, most of the time the adc. You want to flash -> aa -> q -> aa. This mostly results in a kill. If you don't have flash use your q instant.
The reason you want to flash tho, is that the enemy has less time to react and you are pretty much safe to get 2 aa off. Most of the time the enemy will flash away, so you can follow him with your q.
After that you work your way down. Means you look for the next target. Either the next dangerous one, mostly mid laners or the player who is very low.
Because Yis passive you can q after every kill. Make sure to dodge stuns, ults and so on with your q.

Remeber: Flashing on a target gives you a safe aa and the possibility to follow the enemy with your q. In quite a lot of lane ganks, an early flash will get you the kill.

!!I can't remember the last time I used my flash defensively. It's very important to win those late game fights.!!

Some basic things you should know as a yi player:

Q:
- use it to dodge all sorts of abilities, tf gold card, yas tornado, all sorts of skillshots basically
- if you tower dive with teammates, do some attacks then, q to reset tower aggro on the mates. q when the tower shot is flying at u to avoid the damage.
- if you dive and kill the enemy, attack the tower to get your q off cooldown, then q to a minion to get out
- you can ward and jump to camps with q as an escape, for example krugs
- try to predict enemy flashes to follow them, so dont q instant
- sometimes its better to use q on a minion or another player so the bounce kills your target. This is when you either cant reach him, or cant get under turret or into enemies.

W:
- use it to take less damage from big hits, for example jinx rocket, ezrael ult etc.
- in teamfights or when low use it to block some dmg, to give yor team time to help u
- when diving turrets use w when turret hits come, to negate some dmg
- use it as an auto attack reset when really needed. Just aa, then w and aa instanly again

E:
- Just always activate when you fight
- early and for clearing it is the best to activate when double striking, so you get the most out of it

Here some champions, who give us a hard time, early, late or both.

Top:
Early all bruisers in 1v1, mid and late game only if super fed.
-Camille (R)
-Fiora (R, strong in 1v1 when fed)

Mid:
- Ahri (Mobility)
- Malzahar (CC)
- Anivia (survivability, aoe dmg)
- Ekko (CC, R)

Jungle:
- Rammus (CC, We kill us by hitting him)
- Sejuani (A load of CC)
- Warwick
(early and mid a problem, Heal, CC)
- Jax (Tankier and more dmg when ahead, E)
- Udyr (early, CC, tankier)

ADC:
- Vayne (stealth, alpha strike loses
contact when she goes invisible)
- Xayah (CC, R)
- Kalista (only when fed and played very
good)

Support:
- Rakan (CC, R, Mobility)
- Zilean (R, we cant take out the adc
instantly)
- Lulu (Polymorph, R)
- Leona (A Load of CC)

Mid:
- Orianna
- Anivia
- Cassiopeia
- Annie

Top:
- Malphite
- Kled
- Kayle
- Maokai

ADC:
- Xayah
- doesnt really matter

Support:
- Lulu
- Janna
- Alistar
- Leona

Just added a little montage with some scenes of my last games. Sorry for my horrible cutting skills. Just want to give some impressions.
Play Video Play Video Play Video

I think I got everything into this guide, that Is important. If you have questions or requests for things I should add to the guide. don't bother asking.

Thanks for reading and good luck in the Rift.

Btw: The auto attacks of Headhunter Yi and the standard skin feel the best imo ;)