To learn the role, the important ones are Caitlyn, Ashe, and Tristana. They all have good range and/or have good safety/kiting ability for them
For ones that are pretty much a necessity in the current meta: Tristana, Kalista, Jinx (or Vayne), and Sivir.
Tristana is an all around good pick, and has insane sieging ability. Kalista is high damage output, and if you can master kiting, you will be nigh impossible to hit. Jinx or Vayne for if your team needs a late game hypercarry. Important to note that Vayne is by far one of the hardest ADCs to play in an environment such as team matches, so if you want to learn her, then you have to put in the practice to get good at her. Sivir is still very good due to her waveclear and very good engage to get your team's frontliners into the other team.
There are plenty of other pocket pick champs that you could master that can be a good strategy breaker against the enemy, but in order for those to be effective, you have to put a lot of practice into that particular champ (for me, that's Quinn. I can confidently go into a match as Quinn knowing I can win lane or at the minimum stay even, as long as the other person doesn't far outskill me).
Also, champions like Ashe and Caitlyn aren't bad by any means, they just won't do as well with how the game currently is, unless you are building for a particular comp (Ashe can go great with a pick comp for example, as long as you can hit the arrows; but don't expect to carry to hard unless you get fed. Caitlyn can be picked for an early game comp, where your job is to make the other team's bot lane a living hell; take this into something like a Lucian, Sivir, or Vayne, where your high range will have the most impact. Your job in this is to just keep the other bot lane down, relying on other members of your team to be the primary threat.)