Depends on a lot. Generally picking an ADC to counter another ADC is not really that impactful comparing to Top countering Top or Mid countering Mid or Supp countering Supp or ADC. Example, Samira is huge counter to Aphelios. But if you don't have engage supp anyway, then you can't really use why she counters Aphelios. Also if your playing against a full enemy team of peel, peel, peel, then it does not matter that you vacuum counter Aphelios because you wont be able to play the game, so you gotta look at the game broader than just vacuum counters. But generally for botlane as a whole, not just adc vs adc: