tristana or vayne

They all play massively different roles.

Vayne: A late game hypercarry with access to true damage, invisibility and 2 seconds tumbles that empower your basic attacks Tristana: Also a late hypercarry who excels at staying at a safe distance thanks to the highest range auto attack in the game. She also has great self-peel with a jump and knockback Varus: A poking ADC although honestly at this stage of the game you should be using Ezreal, can make a decent mid laner though Sivir: A defensive/supporting ADC perfect for countering that pesky Blitz or Tresh in lane. late game the speed boost you offer your team can really help catch someone off-guard. Damage is not the top priority for her.

tristana is free.