This is my Twitch guide. i haven't been playing Twitch for very long but i have tried many different ways to play him and these are the three that i find you can do well with if you play smart.

Pro

Causes fear in many lanes because you might be stealthed in their lane
Very good 1v1 because of stacking poison true damage as opposed to Teemo who has magic damage which is stopped by magic resist
Expunge is very powerful harass especially if you stealth then get a lot of poison stacks to pop with expunge
Extremely powerful in team fights because of ult and having an AOE slow and an AOE damage skill
Amazing lvl 1 ganks in mid or solo top lane especially with teemo so it can look like a 1v1 to the mid champion but really it is 3v1 for an easy first blood

Cons

Requires skill at positioning yourself in team fights otherwise you will get focused and destroyed
Fairly easy to play but very hard to master and carry
Very squishy

Pros

Extremely powerful late game with damage equal to AD twitch but with much more tankiness for survivability
Can kill tanky dps champs extremely easy with Bloodrazor, malady, and wits end procs
Those three hybrid procs also affect his ult which means in a team fight he can do 4% of every person on their teams health and reduce all of their magic resists with his malady while healing from all 5 of the enemies hit

Cons

Still easily focused and destroyed in team fights even with the added survivability if you don't get proper positioning
Even slower early game then AD Twitch
Requires lane partner with very good early game (for example Garen) in order to have enough money for items and be fed for late game domination

Pros

Very Good map control.
Very powerful ganks because of stealth, huge slow, and expunge as a finisher if they flash away
Early ganks are very successful because of no counters with oracles and vision wards
High lane presence causes many people to play careful even if you aren't there just because you might be which can save many teammates by making them think twice before tower diving an ally

Cons

Ganks aren't as effective later when they have oracles and vision wards
Very slow jungler
Requires team work for success
Difficult to play as a jungler because of very slow start
EXTREMELY squishy / easily focused

I use Magic Penetration Marks for hybrid Twitch and Armor Penetration Marks for AD and Jungle
Flat Armor Seals for a tankier early game on all builds especially Jungle Twitch for the early game armor
Magic Resist Per Lvl Glyphs on all for some late game survivability because you start with enough base Magic Resist to not need them to be flat
Finally Armor Penetration Quints if AD or Jungle and Move Speed Quints for Hybrid because flat AP isn't very useful on twitch because he has one move that scales with AP, Magic Penetration is also a choice if you prefer that but i personally like Move Speed Quints on him because he gets no items to increase move speed unlike AD and Jungle

I take critical chance and improved exhaust on ad twitch then get CDR and 15% Magic Penetration for expunge finishing with attack speed and armor pen next. After that i get 1 in AD mastery with 3 in critical damage then finish out the tree. in utility i take reduced death time, extra experience and mroe buff duration so when you have red or blue from ganking their jungler on their buff it will last a little bit longer

On Hybrid Twitch i get ap per lvl CDR and 15% Magic Penetration which are basic hybrid masteries
then i put points in good hands, extra experience, longer buffs and mana regen. move speed and more CDR with improved flash are next followed by finishing out the utility tree

On jungle twitch you take near identical masteries to AD twitch except you take improved smite instead of exhaust obviously and move a point from crit damgage to flat ad for better early game jungling. then i take 6 extra armor from defense for safer jungling then go utility for death time and a little extra exp.

I hope i dont need to explain both AD and Jungle because they are they same build just a wriggle's lantern at first that changes to a blood thirster later so i will only be explaining the AD not the Jungle.
So i start out with a Doran's Blade on AD twitch for even better 1v1's in mid or 2v2's in a duo lane. Next i recall once i have enough for Mobility Boots. after that i just go back whenever i am low on mana or health or once i get to 1650 for a B. F. Sword. then i get my Infinity Edge and a Phantom Dancer. then i get a B. F. Sword and a Negatron Cloak which gives me a little survivability. then i finish my Bloodthirster and my Banshee's Veil. lastly i get a Black Cleaver and change my Mobility Boots into Mercury's Treads.

for Hybrid Twitch i usually start with a Dagger to build into a Malady later and for some nice attack speed early game for poison stacks. then i go back once i get 1350 so i can buy Boots and Madred's Razors. then i finish my Sorcerer's Shoes or Mobility Boots and get a Malady then go for Madred's Bloodrazor. after that i get a Hextech Gunblade for some survivability. finally i finish my build with a Wit's End for some much needed magic resist and an Aegis of the Legion as a defensive item for health, magic resist, and armor.

Deadly Venom
Twitch's attacks infect the target with Deadly Venom, which deals 2.5 / 5 / 7.5 / 10 true damage each second, stacks up to six times, and lasts up to 6 seconds

This is the ability that sets up all your others. without attack speed this passive is not being used to its full potential. also when its early game and someone is close to death and u get stacks on them and they try to run, let the poison do a few ticks before using expunge. that way they will take maximum, damage and can mean the difference between a kill and making someone recall.

Ambush
After 1.25 seconds, Twitch becomes stealthed for 10 / 20 / 30 / 40 / 50 seconds. If twitch attacks a unit while stealthed he gains 30 / 40 / 50 / 60 / 70% attack speed for up to 10 seconds, depending on how long he was stealed before attacking.

this skill is why on ad twitch we can rush an infinity edge. without this twitch has pretty low attack speed. max this second because it makes your ganks so much better but you need expunge in order to get the kills early.

Venom Cask
Slows nearby enemy champions' move speed by 30% plus 6% per Deadly Venom they have. 2 / 2.6 / 3.2 / 3.8 / 4.4 second duration

this skill is useful but should definitely be maxed last for the one reason that the only thing improving with levels is the duration. it doesn't increase the slow % it just lasts half a second longer. this makes it better late game when you already do a lot of damage because slowing someone a lot when you cant hurt them is pretty useless.

expunge
Deals 30 / 60 / 90 / 120 / 150 (+ 100% of ability power) magic damage to each nearby posioned enemy, plus 20 / 30 / 40 / 50 / 60 additional damage per stack of deadly venom applied to them

this is your main early game damage ability. the reason is it basically scales off of attack speed because the more poison stacks the more damage. it is also an amazing finisher which greatly improves your ganking ability when they blow flash to get away and you just finish them with an expunge.

Spray and Pray
Twitch gains 15 / 25 / 35 attack damage and his attacks become long range line missles for 12 seconds or 5 / 6 / 7 attacks dealing 100% damage and poisoning all enemies hit

this is the ability that makes twitch even more godly in team fights. besides the extra ad it also makes you deal damage to almost everyone on their team depending on your positioning. on hybrid twitch it is even stronger because it deals on hit effects like 4% of their max health to all 5 enemies which essentially multiplies your damage by 5 if they are in a small group or a line. sadly many times after the first time you ult in a team fight and destroy them they will start spreading out or buy an oracle.

For jungle twitch there are many different paths but i find red buff start and blue buff start to be the two best paths. that is because both help your damage output and increase your jungle speed. my personal favorite is the blue start because it allows you to save red for when you want to gank which is around the time you get your wriggles and possibly boots of speed. if you start red you should gank earlier so it doesnt wear off before your first gank. after the buff start you just do a standard route. if blue start you go wolves => wraiths => golems => gank or back to new wolves => wraiths => golems => recall. then you quickly grab red buff and gank whichever lane needs it or has the best opening for one (i.e. only one left in duo lane or solo top pushed very hard). after that jungle till you can get Mobility Boots then just gank whenever possible while jungling between ganks to regain health. the red buff start is the same exact thing except that your first run through when you only have cloth armor is just backwards and that you must gank earlier in order to make use of the red buff because you will not get it for a while. after you get your Wriggle's Lantern and Mobility Boots you basically do the same thing as laning twitch except that you jungle instead of farm when you cant gank anywhere.

i hope you succeed with all the builds and find your favorite. AD has the best early game of the 3 while Jungle has the best mid game with Hybrid having a weak early game presence but an amazing late game team fight presence. this concludes my AD/Hybrid/Jungle Twitch guide. one last thing, dont forget to vote my guide up if you like it and post any questions concerns or advice below and i will try to answer as quickly as i can.
EDIT: i would like to give credit to Deadskull and his twitch jungle build for introducing me to jungle twitch, otherwise i would have never thought it would even work
i would also like to give credit to Ironbridge and his hybrid twitch build for basically the same thing as Deadskull's jungle build
these are the links: Deadskull's Jungle Twitch Guide
Ironbridge's Hybrid Twitch Guide