-Formatted so the guide is easier to read.
-Added images for runes and items to make the guide more helpful to those who dont know the items/skills

As in all guides all items are purely situational, so if you are here to simply grab an item build, you may end up being disappointed. The items you get MUST depend on several deciding factors when you play.

1) what does the enemy team have and how can i build to counter it (adding more life, MR, ATS, AD... e.t.c)

2) what do i need to add to this champion that will coincide with his skills and attributes to make him/her a formidable oppenent

I have decided to make this guide for those of you who would like to show skill, timing and proper countering strategy make YOU the best master yi on the field and that its not all about a "right click to win" strategy. Master yi is most definitely an "easy" champion to play, but a hard champion to completely master.

As for the runes i have decided to pick, they are for a DPS strategy BUT you can use a variety of rune set ups. You can add ability power glyphs , atk speed /dmg marks , dodge seals /armour seals .

You can ultimately change the set up to change master yi from a DPS to a damage soak to a hybrid. IF YOU KNOW you are going hybrid i would suggest atk speed marks , armour seals , ability power glyphs and atk speed /ability power quints .

As for using yi as a complete caster you would have to rely solely on alpha strike for dmg and if you decide to go this route YOU WILL NEED atk speed items that ALSO give you ability power like nashors tooth . To go with the caster yi route i would suggest atk speed/ability power marks, armour seals, AP glyphs and quints.

As for masteries again you should know before the loading screen which yi you want to be

Hybrid 21/0/9

AD 21/0/9

AP 21/0/9 or 9/0/21

Both hynrid and AP can be an amazing dmg soak champion
set ups vary for each style. and the 21/0/9 you can focus on magic or damage and can be useful to utilize the masteries associated with the play style.

it is also possible for hybrid to focus more than the 21 points in offense so that you can maximize both AD and AP dmg, but in return lose the benefits of having points in utility.

I try to customize my skill tree for each battle depending on the opposing team and which playstyle i want to attempt. Knowing your play style will give you a great advantage.
If you are a mana consumer you might want to consider more points in utility.

I really think that the skill sequence for yi is also HIGHLY situational.

Now obviously you are not always going to be with best team possible in solo q
so i would like to take a moment and explain skill order and sequencing to be optimal for your current situation.

Good team
(AD YI)-
(either you got damn lucky in solo or you have a team of your own who you can depend on) IF YOU KNOW for sure your team is good i would start off with a point in wuju style and not alpha strike .
With the item starting out as a vamp septre , this gives you staying power, i have stayed and dominated lane well past lvl 6 with just a vamp septre because wuju style gives you dmg passively and on active USE THIS TO YOUR ADVANTAGE.

Put a point in alpha strike . second then continue to max out wuju style, getting your point in highlander at lvl 6, then max alpha, then meditate getting your points in highlander at every possible lvl.
Usually if you start out strong, and have a laning partner that can stun, you will become an unstoppable force.(people with stuns, slows, pops are all people that will help you easily dominate lane)

Bad team(AD YI)
- now this is where your going to wish you had gone ap instead and where an ap yi truly excels and shows its strength but ill talk more in depth on that later.

IF you have a bad team (arguing in champ select, no clear idea how to build a team, or your in low elo) 1 point in alpha strike first, boots and two pots health and mana.
(if you prefer as this will still make u viable get a dorans ring)

Your strategy
- you will stay as close to minions to get exp and to last hit IF POSSIBLE
(this really starts to suck if you get stuck with a really bad team and they stick you in solo lane and the other team has no jungler. 2V1 can really suck but if u play properly you will be fine.)

LET them push this will be good for you. You can last hit WITH alpha strike AND hit both or one of the opponents.

You will get a point in meditate as your second choice and then alternate between alpha and meditate.
If they tower dive or are more likely to do so i would suggest more points in meditate.

While u sit nice and pretty under tower and meditate You take no dmg while tower obliterates your opponent. Use this skill wisely to bait your enemy into a tower dive.
The only time this wont work is if the enemy has skill stopping power like a stun or a pop that will make master yi stop meditating.
Your best hope at this point is to ask for a lane change or buy 1 of 2 (or both) items, banshee's veil and/or quick silver sash.
banshees will stop the skill from hitting you so this item is preferable, but if you are low on cash (which i assume you will be) get quick silver. it wont stop anything but you can use it to get out of a sticky situation
(note: do not use meditate before getting hit with the skill that will negate meditate, wait to get hit by it, QSS(quick silver sash) out of it THEN meditate under tower)
choosing when to use these skills will define you from the right click to win yi to the master of blades.

AP
-good team, bad team wont matter as much with ap yi, you have to be good with ap yi to try and utilize this strategy, i would suggest doing several bot matches (intermediate) to refine your skills for this play style. Your best skill to maximize on would be alpha strike, then meditate, then wuju style taking highlander at ever lvl possible. (6,11 and 16)

You have a low initial mana pool so try to be conservative with your alpha strike, and know that you will not be able to run in and kill as early as AD or hybrid.
you will need to time your skills to optimize dmg, farm as much as possible when your opponent is out of lane.
early game strategy is to utilize alpha strike for dmg then bait the enemy to tower dive and then meditate.
As for late game you can be a dmg soak and nuke.
In with alpha, (with team) meditate at half life (soak dmg under focus) your team lowers their health why u are focused and meditating, alpha strike again then out.

(please make sure that when meditating all skills that can negate a channeling skill have been used or your meditate will be negated and you will not live long.)

other options are to wait and enter team fight and alpha as much as possible while some one else is being focused.

i apoligize that the ap yi section isnt quite as long, but as i have only played this style a very limited amount of times i dont have as much knowledge on playstyle except early game. usually after i tank the entire team(including turret) meditating, while my team kills them they usually surrender.

NOW my fave style

Hybrid YI
- I usually start out the way you would in the AD style and add AP items to my mid/late game usually after i get these items
berserkers boots , starks fervor , phantom dancer .

then its time for the double whammy item ( sometimes i dont have time to get it because the game is over after JUST those items)

hextech gunblade! DMG AND AP which is exactly what you need, with an added active PLUS! lifesteal. usually after this item the other team gets frustrated by everything about yi- DMG, AP, DPS and damage soak ability with meditate.

Your skills are still going to be lvled up as in AD.

I like to start out as AD because it makes your life so much easier, then adding AP later because they dont expect it.

Starting out AP makes faming and laning so much more difficult so i dont recomend that you do so.

Pro's

high damage output
staying power
skills synergize well
escaping ability
chaser
can soak dmg with meditate
gap closer
pusher
tower bait

Con's

Cant take a lot of dmg
meditate can be negated (SO WATCH OUT FOR POPS AND STUNS)
Easily ganked
susceptable to CC

In closing I would like to apoligize for lack of pictures this is my first guide and have yet to learn how to add features like pictures of skills and runes and such. Please feel free to let me know how i can change that, make this guide better or tell me that it sucks but please give some insightful critisism so that i may take from this first guide my mistakes and learn from them so that i may be a better guide builder in the future.