So for defensive play, generally the best defensive items are this (you should ideally only have one, maybe two if one gives you dmg as well):
: Get QSS vs high CC teams, or if there is a Malz/WW/Skarner, upgrade when you'd ordinarily build lifesteal. You don't need this as often on Twitch because his huge range should mean you should be safe if you position well.
: Really good if there is a fed AP assassin on the enemy team like Fizz, Akali, or Katarina.
: Good against Annie, or other AP burst champs that can kill you before you can QSS.
: Rare item to get, only if there is a super fed Talon/Zed/Rengar and no one will peel for you.
In general,
is sort of a noob trap item atm, but it is getting buffed next patch so perhaps get it over Randy's/Banshees then. Its stats suck for the cost, and the passive is not that good. Only time you get this right now is super late game when you're full build, you can sell your other defensive item for a GA, then once it's used, if the game hasn't ended, you sell it and buy the item you had before again.
is actually a much worse defensive item than you might think and it's really gold inefficient. Really only get it if you're quite ahead. Can be good vs like Garen or Trundle to kite, but I wouldn't rely on it.
The standard build for Twitch right now is:
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(If you need QSS earlier, should get around the time you finish Runaan's) > 