The three strongest ADC's are Ezreal, Graves and Corki atm.

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How is strong? I thought the only reason she was ever popular was because of siege comps.

She doesn't lose lane to anyone because worst case scenario she can just clear the wave with her W+Q.

Strategically speaking, a huge part of the meta right now is dragon control and Sivir's ultimate is absolutely massive for teamfighting out in the open.

Her teamfight in general is extremely good due to both her ultimate and massive AOE.

She's fantastic against the assassin meta due to her spellshield.

She's fantastic with the assassin meta since she makes up for their lack of waveclear. She's arguably the best wave pusher in the game. Consider that ranged waveclear is a glaring weakness of champions like Leblanc and Fizz. Even Zed has to expose himself and use a ton of energy to clear a wave, and he can't completely clear it without autos.