This list is obviously based on personal opinion... But I've noticed most tier lists focus on Diamond Level play, and a lot of that simple doesn't hold true at low ELO where you can't always count of your Jungler or Support to know what they're doing at all. So this champion ranking bears specific weight on a champion's ability to single handedly impact the game, fulfill the expected role of an ADC without a lot of support, and escape ganks.
Draven tier is for those champions that are Dravens.
These champions have the tools in their kit to heavily influence the game even with little to no help from their team.
These champions are all solid picks, but may have some holes in their game and need additional support.
These champions are still good in their role, but may need a more experienced player behind the wheel.
Not Really an ADC tier
Sometimes you'll be in a game, lock into your choice and then find out the maniac who picked Bot at selection is going to play Jungle or Mid or whatever, and everyone else can eat it... Luckily, if you picked one of these champions for top or mid or support, you can actually do a pretty great job as your team's ADC. But you should never actively go into a game looking to play these guys at Bot, or you'll risk tilting your team before the game even starts.