Not Updated For Current Season
Cleanse - this spell is endlessly useful, say ashe shoots her ult at you and thinks you are stunned, you can quickly use cleanse and kill her. Also its the best way to beat a sneaky warwick as his ult becomes useless if you have cleanse.
Flash - is always affective for stealth champs because you can quickly flash away and stealth. Or if you are in team fights flash behind your team when you are targeted and all the melee champs think twice before moving that far to kill you because they will waste time and get hurt.
Masteries: Obviously twitch needs the damage from the offensive side but other than that health regen for early game or extra defence is always useful.
Ambush - 1 point in here at the start for early ganks and scouting who you are against, the attack speed is better than being able to slow the enemy so put points in this after expunge.
Debilitating poison - 1 point in here at the start because the slow can help finish the enemy especially when you have your team attacking at the same time, other than that it doesnt help your damage and only the duration gets affected where at early game this is probably not as much of an issue, the only time you will need it is when they are just about to get under cover of their turret or possibly to help you escape.
Expunge - can do a lot of damage early game and thats why i max this first, as soon as you get 6 stacks just press e and watch them run as you shoot their ***, if they still live the extra poison may finish them off. Its also good as a finisher if they run early because of its long range.
Crit and armour penetration, both are always useful throughout the game and i like to balance my stats as much as possible for them to compliment each other.
The early lifesteal is very important for twitch to stay in lane and keep harassing. The attack speed from malady is also important to quickly get your stacks of poison on the enemy. Also once you have malady it may be best to get lizard buff and start sneaking up on unsuspecting enemies.
Boots of mobility are extremely useful to run around the map quickly getting those kills and to reach the farming areas before all of the minions are dead. You will find these are endlessly more useful than a bit more attack speed, which you wont need if you follow this guide.
The next 2 items can be built together if you wish depending how much money you have available. if you cant buy those expensive items you dont need to save, just spend so you can get the damage in faster, Although you should defiantly get them both before moving on to the next item. the 40% armour penetration and attack speed work well for high damage on those first hits. Black cleaver can put the enemy armour into negative values so the more you attack the more damage you do regardless of their armour, this works well with twitches attack speed.
Once you have infinity edge you will be doing crits for around 700 damage and getting extremely fast kills. and because of your armour penetration and attack speed you will be affective even against tanks.
Phantom dancer boosts your crit chance even more along with giving you a very high attack speed without maxing it when popped out of Q. the dodge chance is obviously helpful too and move speed with your boots makes you the ultimate assassin.
If you do happen to get this far, you might want to swap Malady for stark's, slightly slower attack speed but more lifesteal and armour penetration. also is very helpful for your whole team.
(by mixing attack speed, damage, crit chance and armour penetration you do loadsss more dps than just stacking damage or crit. These items allow you to build up fast early game and are still just as affective late game)
Very often with this build you will find you get scores like 20/5/20, meaning its not uncommon to max out your items quickly. Please test my build and add comments =)