This is entirely based off of your average match. So champions like Evie, although the incredibly good ones will go 67-4 and kill you 25 times in 5 minutes will be lower than it seems they should. This is because Evie takes an incredible amount of skill to get that good with. Each group is roughly in order from most to least annoying.
Edit 1: Corvus has been moved.
Edit 2: Barik has been moved. Ying has been moved.
Edit 3: Raum moved down from SS to S, Zhin Moved up to S+ from A-, Terminus moved up to SS from S+, Seris moved up to S+ due to her high damage and insane healing, Atlas moved up to SS, Evie moved up one, Strix moved down to B, Willo moved up to A+. Vora is coming soon, based off of what I have seen in the PTS she will likely fit into the SS or S- area.
Edit 4: Lot of moving champions around, Yagorath would be much lower but unless you have a proper counter like Burn Monster they're next to impossible to stop right now. She would be higher but she's so team dependent I can't reasonably put her above an S-.
These are the champions you see and instantly know you won't have any fun in that game.
These are the pussies that hide behind a fucking shield for 30 seconds and then kill you when you're reloading.
The champions are easy to learn meaning that even a player with average experience has high annoyance potential.
This is where a champion gets instantly annoying after the first kill because you already know they're about to skill trap you again the moment you get back out there.
These champions are champions you know for a fact are going to be either somewhat annoying they will make you kill yourself angry.
Special spot for the OP supports.
This is where champions are always either going to be awful or absolutely ruin your night.
These champions are the ones that are normally slightly annoying but not enough to really bother you greatly. (Unless they're highly skilled)
These are the champions that aren't normally that bad but can get slightly annoying with the right user.
These are the champions that, excluding those really good outliers you can't really find too annoying.