this is an "all or nothing / all in" build. just enough sustain to keep you going, but more than enough to obliterate all but the heartiest of tanks in less than two seconds, plus with your skills and items makes it perfect for scoring multikills in teamfights.

this build focuses around damage, life steal, attack speed, crit. as such you will be the walking definition of a "glass cannon" capable of doing around 300 damage*5 = about 1500 per 2 seconds + 70% chance to crit for 250% damage = about 65% chance to do 3700 in 4 hits/2 seconds and add in the hurricanes 50% (1875) of that to two other enemies plus if you ult it hits EVERYTHING in a line with his on hit procs

I prefer to take scaling attack damage over the attack damage runes because all too often games go well into "late game" (18+) and the little extra damage every level scales well with this adc. however if you plan on hitting max attack speed you the quint runes as well. High attack speed is rather important.

basic 21-0-9
Good offence, with a few random utilities.
I like to add as much move speed in where I can just because twitch is like the slowest champion in the entire game and every little bit helps.

to start rush the thirster and the hurricane. this will allow for quick farming, and spreading your passive (if you do it right you don't even need to directly hit the enemy champions to get your stacks from your passive and some poke action) and a bit of life steal to sustain. This is good for early game laning, as it should also help keep the enemy team pushed back giving you more room to farm with little worry about trading, and starves them.

next is the infinity edge. this is where you get your momentum going to start steamrolling, this is where you start to carry. as you should already be keeping the enemy pushed back with no problems this just adds insult to injury.

then either phantom dancers and youmuu's depending on the situation you can get either one first. if you start dealing with tanky champs then get youmuu's, if the other squishy's are picking up get the phantom dancers. its a balance of either attack speed/crit or armor pen/crit

final item being the blade of the ruined king. good all around item gives you some decent points into the main stats we want being damage, life steal, attack speed, crit.

as for substitutions I recommend boots and maybe gold income if you have a slow start or a mediocre support, or just don't like getting caught out or whatever

ult every time
contaminate every chance you get
mix ambush and venom cask for everything else in between.

twitch is in fact one of the slowest champs so ghost is very important for both escape and ambushes.

I supplement his slowness with teleport and ghost, being that his spray and pray can outrange towers so a well placed ward can push a tower easily mid to late game, and ambushes all around.

this is my twitch build, it looks and sounds goofy and bad but it rocks. i've had to alter it considerably because of item changes like thirsters passive is now a unique passive lifesteal, no more soloing baron :(.

however this build should allow you to annihilate teams at a time as long as you dont get focused to hard. just enough damage, speed, and lifesteal to keep you going forever, and then you ult to make the good times roll, with your ult and the hurricane (a normally useless item) you lifesteal all the minions and champs and everything else in front of you for insane amounts of damage. If played properly you can make or break team fights in two or three seconds. or you can easily back door turrets (without minion support) and be gone before they can react (great for creating diversions from a mid lane assault on your base at the last second).

I haven't tried it out very much in ranked play yet. as i don't really do ranked much. However I have been trying to recently, and the build works extremely well I'm averaging like 20-5-10 ish area

pros:
1: very high almost maxed attack speed
2: very high crit chance
3: high damage
4: good life steal
I get regular multi kills 1v2+ is common
cons:
1: not enough sustain if you can not outburst them your dead
2: little health so your are EXTREMELY squishy (the very definition of a glass cannon)
3: twitch is one of the very slowest champs in the game roster so escape items and spells are a must because relying on ambush will get you killed as it CAN take up to 6 full seconds to disappear while under attack

as pvp goes he is hard to beat when you ambush them from behind and just burst auto attack.
pve with the hurricane and some damage you annihilate them its perfect.

two strategies come to mind
1: pre fight with ambush for invisibility and an att speed boost, line yourself up about mid party but not to far back, and spray and pray which already shoots through them at huge range (especially if they line up just right) with the hurricane hits literally just about everything on screen in front of you at nearly 2.5 att speed and 300ish damage + very high crits, if IF there are survivors you can usually clean up with expunge as they almost are guaranteed a full stack of poison.

2: pre fight with ambush, get behind them to form a pincer attack and after the engage you come out of nowhere with your spray and pray and catch them very off guard and confused, this is a very risky maneuver but it has payed off for me numerous times. ONLY do this maneuver as a hail mary pass, or if you know that you can take out either all/most of them or do a massive amount of damage so your team can clean up easily. WARNING THIS MANEUVER WITH GET YOU FOCUSED VERY QUICKLY

ambush: a blessing and a curse. go in invisible catch them off guard, + more att speed
however if you are being attacked it can take up to 6 seconds to vanish and that is long enough for you to die several times over. if done correctly allows you to juke with ease.

toxic cask: ehh it slows and adds two stacks of poison, its kind of a ok early game poke,not much REAL use it does come in handy at times though

expunge: the perfect finisher very good range, does decent damage, great for escaping champs with VERY low health,

passive toxic something on another: you can't outrun poison so if you don't get your expunge off and they are very low health there is a fair chance it will still kill them. this with something like ignite can be very deadly to a champ that thinks they can flash, and jump/hop/skip/dash/etc out of range almost instantly. plus the lols of dieing and still getting a kill/s like 5+ seconds later, not to mention the assists you get for the extended damage coverage

spray and pray: add extra extreme range to your auto attacks (enough to outrange towers) plus it shoots through enemies in a line albeit at reduced damage. GREAT for team fights and killing those pesky mages and other squishy's that hang out in the back destroying your team, all from the safety of the sitting on the far side of the fight through your team, walls, jungles, basically anywhere they can't hit you.

your first two items covers about everything, decent damage,good attack speed, and you hit several at a time so there is not much of a need to last hit for very long as your auto attacks annihilate minions rather quickly.

however while laning vs champs that have good gap closers and stuns etc you will want to keep at nearly max range at all times and keep moving as you are VERY SQUISHY