Recently Twitch got updated. Most people think he's useless now. My build will prove otherwise.
They just changed him, they didn't buff or nerf him. I think it's still very enjoyable to play Twitch.

Pros
- Invisability (helps to escape too).
- A lot of damage and one of the best gankers
- AoE-damage ultimate; very strong and deciding the teamfights.
- Scouting
- Fun to play


Cons
- Squishy (masteries in both magic resist and armor along with runes like in my build will help).
- You will always get focused (because the old twitch used to be focused down right away).
- Oracles & vision wards will be a pain.

MARKS:

Just get either attackspeed marks () or armor penetration () depending on what runes you have.

SEALS:

Equip armor seals (), otherwise you will be too squishy. Believe me, this really helps you die less.

GLYPHS:

Equip the magic resist glyphs (), these will really help you.

QUINTESSENCES:

Equip either armor penetration quintessences () or attackspeed quintessences (). If you have neither then equip health quintessences ()

My setup (working pretty good, please comment if you know a better one xD):

For Summoner Spells I prefer:
- Ghost(), this spell is both good for escaping and hunting/ganking.

- Exhaust (), very handy whilst ganking, or you could protect yourself by exhausting the opponent who is chasing/fighting you.

Other Spells you could use:

- Flash (), always good for escaping.

- Ignite (), to get a little bit extra damage during your ganks.

Deadly Venom (Passive): Twitch's attacks infect the target with Deadly Venom, which deals damage each second, stacks up to 6 times, and lasts up to 6 seconds.

Ambush: Twitch hides in the shadows, waiting for a nearby enemy. If he attacks, he will gain bonus attack speed. Great skill for ganking, at first it's only for 10 seconds but at level 5 it's 50 seconds. You should make sure you are using your attack speed boost for as long as possible by waiting to get visable again until the timer (almost) wears off.

Debilitating Poison: Infects all nearby champions with Debilitating Poison, slowing their movement speed. Usable for running ennemies. You should first shoot 2/3 times and then activate this skill.

Expunge: All nearby enemies secrete Twitch's toxic venoms from their bodies, dealing damage for each stack. This is your main damage skill, just keep stacking poison on ennemies and activate this skill to kill them. THIS SKILL HURTS

Spray and Pray: Twitch closes his eyes and fires his crossbow, spraying powerful piercing arrows ahead of him. Attacks become long range and the attacks pass through units. PERFECT for teambattles:
1) Open up with your ult (R); make sure you hit most of your opponents
2) After 2/3 shots, slow your opponents using debilitating poison (W)
3) After 4/5/6, shots use expunge (E); you will deal massive damage on multiple ennemies.
4) Pick off what's left, probably nothing =D.

CORE ITEMS:


These will make you have a lot of attack speed and critical strike. They will make you move very fast too (480 movementspeed)

Attack speed + Venom + Expunge = Damage (the Twitch-equation)

After you got that, build it to:

Twitch is now inescapable!

Then you should pick an item that fits your situation:

- If you die a lot due to magic damage, get a Banshee's Vail.


- If your opponents keep stacking armor, get a Black Cleaver.

Mostly I prefer to lane with Twitch. In laning phase you got two goals to achieve:

1) Killing a lot of minions, do this by last hitting, last hitting is very important. If you don't last hit, your opponents will be forced to towerhug. In that scenario you and your fellow laner will be very vulnerable for ganks (by their jungle for example)

2) Getting kills, you do this by using your invisibilty skill. This will give you a lot of attackspeed when becoming visable again. Let your poison stack on your opponent and use expunge to finish off.

If your team doesn't have a jungler, then get red and blue.

During laning phase you could gank mid, just get there and position yourself behind your victim. Open up with a regular attack and put some exhaust on him. Expunge when he's out of range. You could gank their blue too if they have a jungler. Junglers often get low hp when getting blue buff early game, a perfect victim! Positioning is the key to a succesful gank!