Kha'Zix

His invisibility will run out your highlander, can kill you with ease early-mid game if meditates on CD.
Win Condition: Don't feed him early on, take sweeper, beg lanes to control ward bushes, and third party every fight.

Wukong

Wukong's new ultimate will disrupt you and your whole team from doing anything.

Shaco

Probably worth banning, and honestly one of the most annoying champions for Yi to play against.
1. Whenever you fight him, he will use his Q, a flash combined with invisibility, preventing you from using abilities on him.
2. His boxes will fear you, and if in highlander, you will run halfway across the map.
3. Although in a 1v1 scenario it is not difficult to tell the clone from the real one, you will NEVER be able to count on all 5 teammates to do the same.
Win Condition: Ban or Fight him after he has used his Q and R.

Jax

Jax is the popular Yi counter, and for good reason. His sustained damage is almost as high as yours, and his counterstrike will negate all of your damage.

Win Condition: Jax is manageable, but you must dodge his counterstrike. Meditate as soon as he counterstrikes, wait about 1.5 seconds, and then Q to maximize the probability of you dodging the E. Most Jax players will use most if not all of their E duration, but if you are too predictable, by the second or third fight he will stun early.

Rammus

Another Yi Counter-pick, Rammus relies on opposing a primarily AD team comp where he can stack armor.
Win Condition: Rammus is not that bad but you MUST take certain items and change your play style.
Take Ignite Smite. You can't run from a Rammus if he has his eyes set on you. His Q is faster than ur R and he runs flash as well. The ignite will help combat power tremendously and deal true damage to him.
1. Right before Rammus Crashes into you, activate true damage E. Otherwise you will kill yourself from his armor and thorn mail.
2. Play the early game safe, but understand that rammus rushes cinder hulk, rather than armor, and thus capitalize during this timeframe with your team, and use ignite to counter-gank him when his abilities are on CD.
3. Late game, you should be able to win by ignoring him in fights by purchasing QSS, focusing the squishies, and then teaming up on the Rammus with your remaining team mates.

Zac

Easy to kill 1v1, but if your lanes are pushed with no vision, he will gank them endlessly, snowballing them. Win Condition:
You CAN NOT keep up with his ganks and just have ask your teammates to survive till late game. Beat him in farm, and do your best to track him and predict his next gank target .

Vi

Insane early damage, CC, and shields.
You will start to win fights after you finish your Tabi's, Bloodrazor, and Rageblade.
Win Condition:
Early game, never fight her alone. Stall out fights with W only after her dash is on CD.
Stick next to your backline in team fights in order to kill the Vi when she R's your ADC.

Warwick

Yet another champion that destroys you early game! This one has insane early game damage from his W passive, stickiness, and an R and Q that he can heal half his healthcare with.
Win Condition:
Dodge his fear with your Q, never 1v1 him before Rageblade, and purchase Mortal reminder. Ignite might be worth taking here to reduce healing early.

Diana

Diana is a huge threat to many mid-laners, but not Master Yi. She does less sustained damage than you with her auto attacks, and has an easily dodge-able ultimate. If you alpha strike during her R, you will always win the fight.

Mordekaiser

Annoying until Bloodrazor. Has a chance of killing you until Rageblade at 16-22 minutes. Long story short, you can dodge his Q, and heal faster than he can damage you with basic attacks and passive. Don't waste QSS on his ult. It's free 300 gold.

Gragas

His R does loads of damage, and knocks you back pretty far. However, that's almost never enough to lock you down.

Ekko

Ekko is an interesting matchup, and the interactions with his abilities are somewhat inconsistent. You can Q the stun easily, and alpha when low to follow him in his R, but the rest of your team probably will not be able to handle him without. you there.

Olaf

His passive makes him nearly impossible to kill early-game(2+ attack speed level 3 is insane). Take ignite, and gradually you will see that he becomes a less effective version of you mid-late

Udyr

Although rarely played, a good Udyr is a pain in the ***. You can't dodge his abilities, he can run almost as fast as your highlander by pressing a basic ability, and he has. point and click CC, and 10 other early game stat buffs that make him hard to deal with early-game. He WILL invade you and if your lanes do not feel like helping out, there isn't much you can do. Not worth banning due to his play-rate. Just farm as much as possible, take free ganks, and hope late game will save you.

Twitch

Twitch can cheese gank at level 2-3 very easily, but unless megafed with peel from 2+ champions on the other team, focusing him down should not be a problem. He has no mobility, and can not kite you during highlander. If you stick to him, you can also attack and Q him during his stealth.

Tryndamere

Without R, he is nothing. Obviously, if you just stand there and auto attack him during his R he will win every time, no matter the items. If the enemy adc builds crit(Almost always), and/or he is on the same team as a Yasuo, build Thornmail and Streak's Gage. Meditate during his ultimate and you should win.

Kayn

Kayn loses 1v1's against you at all stages of the game unless extremely fed. Watch for raptor cheese level 1.
Blue Kayn: With Tabi's and death's dance, he is no threat to you at all. Stick near an ADC to pick him off after he R's.
Red Kayn: He is tankier and harder to kill. Dodge the knockup, focus the squishier targets before turning to him.

Nocturne

Save alpha to dodge his fear. Wins early game fights due to his passive attack speed, as well as his Q. His R is a great ganking tool, but bad when trying to team fight.

Volibear

Nunu & Willump

Your kit counters every aspect of Nunu's. Slows don't work during highlander. You can alpha both the root and the ultimate. The problem is that Nunu spamganks, and can steal objectives with ease. Play around these and win.

Trundle

Before 6, it is hard to fight him, but honestly he can't really do much except auto-attack and Q. Watch the early game and you will be fine since you can not be slowed in R.

Amumu

Amumu is weaker than you early if you countergank him. However, it is almost impossible to carry when against one, as he will simply R, prevent you from using Q, and chain CC. QSS needed.

Kindred

Very easy matchup, as she is a ranged champion with no CC. She will R her and her team when they are low. Make sure you switch targets during it, make as many low HP as possible, and Q right after her ultimate expires.

Shyvana

Activate E, and alpha when she R's away. Easy matchup.

Rengar

Meditate his ultimate damage, Build deaths dance to negate most of his lethality. Early on, do not fight him in bushes(obviously). Although he has very high burst, he has little sustained damage and no escape other than an empowered W. In team fights, just focus on killing the enemy backline, as Rengar will also dive your backline.

Evelynn

Her agency in the game depends on how fed she is. If lanes blindly push up without wards and feed her kills, she can easily one shot everyone. However, if burst is on CD, she is a walking cannon minion.

Fiddlesticks

His new fear on his ultimate is obnoxious, and requires a QSS. Difficult to handle If fed, because he can R in the middle of your team and use Zhonyas.