Marks: The only real options for are Attack Speed, Crit Chance, and Armor Penetration.
Seals: I'd recommend Flat HP or Dodge, but if you really must you can go Crit Chance/Attack Speed
Glyph: I'd recommend Flat CD Reduction, but if you must, you can go Crit Chance/Attack Speed
Quints: I'd recommend Flat HP, but you can go Attack Speed/Crit Chance/Armor Penetration.
Attack speed is nice because it increases your DPS and helps to stack poison, but Twitch already has two attack speed skills.
Crit Chance is nice because it's powerful throughout the game, and even more so if you get an Infinity Edge
Armor Penetration is extremely powerful throughout the game, and is often underestimated. If the enemy has 20 armor, and you have 17 armor penetration, that's 16.5% increased damage. Flat Armor Penetration is very effective versus enemies with approximately as much armor as you penetrate, which is relatively true early game. Then late game Last Whisper combines well with flat armor penetration, making it effective throughout the game.
Flat HP is *always* nice, but especially early game. It gives you more survivability, which in turn allows you to be more aggressive, and makes it much harder to gank you. (As a low HP champion, a small amount of HP makes a big difference.)
CD Reduction is always nice, as well, and since the crossover is level 13, Flat CD is in my opinion generally better.
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