There's no real best. Cait for example doesn't fall off lategame compared to say... Rengar. She's better than Ezrael (who doesn't scale that well late despite what you're saying, he's valuable late game due to his escape). However, she's not a hypercarry like a Vayne or Kog'Maw. In comparison for other ADCs, she's more of a midgame threat, like a Miss Fortune or an Urgot. Her escape, CC, and other utility make her very nice in all stages of the game though.
Not the best ADC (as IMO there isn't one) but certainly a great one and a force to be reckoned with.
Edit: Agreed, Jamaree.