Easily Rocket League. It's always been toxic but it's been much worse since becoming free to play.
Rage quitting
Malicious compliance (AFK, accidental own-goals)
Team mates playing for the opposing team entire games
Passive aggressive quick chats
Overtly aggressive free-form chats (being chastised over every tiny mistake)
The game's competitive ranking systems makes it so each win and loss matters, and people often take their vitriol out on their own team before looking inwards.
lol, I love rocket league and I can't deny this, but there is also a thing called confirmation bias as I don't play other games to verify.(Fighting game for example can be pretty toxic as well.)
Yesterday, I have a casual 2s match where the opponent are really good, they have maybe gc2~3 mechanics(judge from their touches, consistency and boost controls, way better than my usual C3-GC1 encounters) and just trying to get clip shots on me and another random. We were originally doing fine, BUT, for some reason, after they scored the 2nd goal with some flip reset stuff. My random team mate starts to trying to "show off" with ceiling shots and his own flip reset thing. Like I don't mind if you can actually pull that off. But he mess up or simply not mechanic enough with that which leaves me defending 2 of them with comm, it's impossible for me to work against 2 GC let along 2 GC2-3. Somewhere after we lost 4-1 and he had a good shot from my blocking a pass, it's literally just catch and drive up to score cause the opponents are still "falling" from they mid-air prejump stuff. No, he has to score a flip reset so he push to mid boost and then did his setup. You know how fast they can be so by the time he is off wall the opponents are back and one in goal on on wall. Then me just watching him doing his stupid flip reset attempt.
And then he missed the ball, I let out a long "urrrhhhh" sigh in VC don't know if he hears it or just too shameful, he throws the FF after my sigh and I gladly accepted it. I usually don't even FF if there is a toxic play on my side cause I like keep them around so they don't ruin other people's game, if they quite they eat the ban for 5 mins, so take one for the team I can manage. But I can't just keep up 1v2 in that situation. And to me that play style is more frustrating than dealing with toxic players.
Every game community has some rotten apples but RL has by far the highest percentage of 'em in any game I've played. I get the urge to play RL a few times per year, but it only takes a few games (be it casual or competitive) to put me off again.
I played CSGO for a decade l or more. Got into rocket league and found the community far worse in rocket league. Counterstrike is like a chatroom with guns. Rocket league is wanna be pro level play and blaming teammates when you can't make the play you wanted to.