Hello!

This is my build for a twitch. I hope you will like it.

(My english is not very well so sorry for any mistake.)

Armor penetration mark for doing more true dmg
Hp per lvl seals for some hp, we can't have low hp in late game
Attack speed glyphs for more speed
Hp Quintessence, not being too squishy

I pick 23/6/1

you can have 21/x/x but i think you can put some points for 15% magic pen. for expunge.

6 def. to more survi and 1 utility for better ghost.

You are looking at this items and say two question, "tiamat? for what?" and "Crits? why not more lifesteal?"

Timat is verryyy good item for twitch, when he use ulti, every attack( penetrate shoot) deal tiamat dmg. When you have all of items and you hit with crit yours timat will do approx. 150-200 dmg, now look you hit all of champ with your ulti when they were on group, you will attack them with timat by 1000 EACH PERSON.

Crit is very important too. You will have more dmg on tiamat passive, and twitch ulti have one good thing. if the first hit was Crit (on ulti), every person will get 100% chance crit. It is very powerfull!

20% lifesteal will keep you alive in teamfight's

I chose Exhaust and Ghost. I think it's the best for a twitch.
Exhaust can help you with ganks, reduce dmg and also you can escape with it.

Ghost its for escape or run fast to gank someone ; >

You can also pick:

Flash - great escape from teamfight

Ignite - to kill person who escaped