This build shows you how to take Yi's kit to its maximum potential.

AD mark and quint runes for stronger Q and auto-attacks.
Defense for reducing incoming damage because you want to stay alive in order to be effective.

21 Offense with crit. spec. so that you can deal the most damage possible with your kit.
9 Defense because you can never have enough damage reduction.

All AD and auto-attack damage oriented items to make you a melee ad carry. Buy items as needed. If you keep getting focused down too hard during mid or late game, then don't hesitate to start building your Guardian Angel. If you're up against an all-AD or all-AP team, then definitely buy an item that focuses on stopping them, like a Randuin's Omen or Mercurial Scimitar.

Max your Q first because it's your most reliable damage, and your runes will give it more damage.
Max your E second because it's your next offensive ability, and by mid and late game you should have at least two or more of your offensive items to be able to benefit greatly enough from it.
Max your W last because it's an unreliable defensive skill that can be stopped by any moderate to hard cc.
Max R every chance you get.

Ignite for the extra damage, heal reduction, added the 5 ad/ap bonus from "Wrath" [the offense tree in your mastery page].
Exhaust and Ghost are the only other two spells I'd recommend because they help you close the gap for auto-attacking or open it for the escaping.

Build high damage, sustain, mobility, and some tankiness if needed. Kill fast, or be killed fast.

Hyper-Carry damage potential.
Lane-pusher.
Scales well.

Squshy melee is squishy.
Needs allies to provide cc in order to help him carry.
No kit-based cc whatsoever.