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I've discovered that I love that little rat *******.
Normally I start with 1 point in Ambush (for scouting), then maxing Expunge with 1 point in Debilitating Poison at level 4, level 3 Ambush, max Debilitating Poison, max Ambush. I grab ult whenever possible.
First I usually go with Executioner's Calling. Lots of Twitches seem to go straight for Malady and I can't for the life of me figure out why. One Phantom Dancer and his aspd is already massive. Executioner's Calling has good crit, lifesteal, and that nasty anti-heal poison which is fantastic early game for numbing their Heal and making every Venom tick count (gimped regen.)
Second is boots, usually go for Berserker's Greaves for obvious reasons.
Third is Phantom Dancer, because the more stacks of poison I can get on someone as quickly as possible midgame means faster Expunge and guaranteed kills. Without it, it seems like I can't really make use of Twitch's abilities when they're really needed. The massive crit boost and movespeed buff are fantastically useful, and even the dodge rate helps from time to time (YI.)
Fourth and fifth are the carry staples, Infinity Edge and Black Cleaver. Between them, I can annihilate 3 champs at once if I can catch them with their pants down. 70% crit, stacking armor debuff, and gimped Heal destroys just about every champ in a 1v1 so long as you play smart and know when/how to initiate (exhaust carries NOT YI, surprise ult at a friendly tower when they think it's unguarded, stalk mages until you get breathing space etc.)
Any items after this don't really matter, if you get this far, I know what I need (bloodrazor, bloodthirster, frozen mallet) to keep the blood flowing.
Summoner spells usually Exhaust and Teleport, seems to work best.
End-game I usually have 1.5-1.6 base attack speed and completely ridiculous damage if it all goes well. The key is definitely choosing the terms of engagement (which bumping Ambush up a bit really helps with, you can stalk champs outside of Oracle range for quite a while before choosing the moment) but so long as you do that well, it doesn't seem like you can lose. With a good initiator (<3 malphite/alistar) Twitch is just utterly unstoppable. How can I make him more unstoppable?