Not Updated For Current Season
I'm quite new to twitch and this is the build i'm experimentig with.
I find that although boots of mobility is nice your main task is to kill, so i go for extra attack :)
The last item in the build may as well be Bloodthirster, Black Cleaver, Razor or another infinity :)
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 8 seconds.
Ambush - Twitch's stealth ability. Shooting a target while stealth will increase his attack speed significantly. This ability will always be used for scouting, ganking, and so forth.
Debilitating Poison - Twitch's poison that slows your target by 30%+8% for each stack of deadly venom you have on the target. This is a great support ability for yourself or your team. If your opponent begins to run, using this ability will increase your chances of catching up to them. This ability can be used on multiple targets within a certain radius. An example is if there was 100 enemies within your radius it would affect them all.
Expunge - Deals instant magic (30 / 60 / 90 / 120 / 150 based on level) damage. The more stacks of deadly venom you have on your target, the more damage you will do. This ability is your main attack for finishing off your opponent. I suggest you have 6 stacks of deadly venom on your target before using it to dish out the most damage. Of coarse if your target is very low, use it when u can before he escapes. Just like Debilitating poison, it can affect multiple targets when used in a certain radius. This is why Twitch gets a lot of multi kills.
Spray and Pray (Ultimate) - Twitch's Ultimate attack. It increases your attack speed and shoots poisonous arrows in your frontward radius infecting your targets with Deadly Venom. This ability is extremely useful in a team fight in which u can infects the entire enemy team with Deadly Venom. Expunge afterwords to kill some killing blows!
Exhaust - This is my favorite because of its slowing effect and most of all its blinding. If your chasing, retreating, or going 1v1 with a melee champion always use this. This will give u the advantage against a melee champ to dish out some damage before you start to get hit after exhaust runs out.
Ghost - Increases your movement speed by 32% + 8% if you added Haste in your Utility masteries. You also avoid all unit collision which great when your retreating to chasing. Use this spell if your enemy begins to run or you need to run. Combine it with stealth for a quick get away.