Who do I buy? Taliyah vs Lissandra

And any assassins. She hard stops most of their play patterns and excels at turning ganks/dives.

I think Taliyah has a higher mastery curve and a stronger overall game presence. Lissandra spikes in mid game and slowly tapers off, and brings a higher level of utility. I would go ahead and grab Taliyah and start putting the time in to master her. Lissandra is a powerful arrow in the quiver, but in most seasons she's been a situational pick. Never a bad pick, but not great into blind matchups.

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Taliyah because Lissandra needs some buffs

Until Lissandra finally gets a new passive or has the speed on her claw increased I would pick Taliyah. You would seriously have to be high on xanax to get hit with her claw it's so slow. She is an all in combo mage like LeBlanc but lacks the mobility that LeBlanc gets because her claw is just too damn slow.

I disagree - all Liss has to do is simply press R on a stray ADC and they are dead, no questions asked. You don't have to hit anyone with her claw - if you fire it over a wall from the fog of war, they won't see it until it's already too late and you're on top of them.

This is despite her balance state - I do believe she is weak atm, numbers wise, but you seem to believe her kit is the problem when she is a far better teamfighter than LeBlanc is, and is actually pretty close to a direct counter to her in every way.

https://s3.postimg.org/ugy4qvfoz/lblanc.png