London System.
Most Toxic Players Based off Opening Choice

London System.
Definitely up there. Those guys think that they can just play the same moves against anything and think they are good. Back when I was 1400, I would still trick London players with 2...h5. Just play a Queen's Gambit

The annoying d*ckheads on lichess who play 1.a4 2.Ra3 3.Rg3 against everything and win anyway. It seems terrible but is annoying to face in a blitz game. If black moves his dark squared bishop anywhere then g7 is hanging, and if he plays g6, then they go for h4-h5. I had no idea that this was even a thing until about ~2300 blitz, when everyone started playing it vs me.

Caro-Kann has to be the most toxic
Whaaat?
They are so toxic against anyone that plays the French and it's very mainstream

London System.
Definitely up there. Those guys think that they can just play the same moves against anything and think they are good. Back when I was 1400, I would still trick London players with 2...h5. Just play a Queen's Gambit
If you really want to take down the London System, play the Modern Defense.
1.d4 g6 2.Bf4? (This move is fine against 1...d5, 1...Nf6, or even 1...f5, though less effective in the third case, but it is outright bad here) 2...Bg7 3.e3 d6 4.Nf3 Nc6 5.h3 e5! 6.Bg3 (6.Bh2? -/+, 6.dxe5? dxe5 7.Qxd8+ Kxd8 8.Bg3 -/+ [8.Bg5? f6 -+, The Bishop is dead) with a slight advantage for Black (=/+).

The annoying d*ckheads on lichess who play 1.a4 2.Ra3 3.Rg3 against everything and win anyway. It seems terrible but is annoying to face in a blitz game. If black moves his dark squared bishop anywhere then g7 is hanging, and if he plays g6, then they go for h4-h5. I had no idea that this was even a thing until about ~2300 blitz, when everyone started playing it vs me.
People win 51% with that garbage. The issue is you get tilted so you try it out yourself only to lose more games.

Caro-Kann has to be the most toxic
Whaaat?
They are so toxic against anyone that plays the French and it's very mainstream
I play the Caro, and I think the French is a very good opening.

Caro-Kann has to be the most toxic
Whaaat?
They are so toxic against anyone that plays the French and it's very mainstream
I play the Caro, and I think the French is a very good opening.
Hmm

Caro-Kann has to be the most toxic
Whaaat?
They are so toxic against anyone that plays the French and it's very mainstream
I play the Caro, and I think the French is a very good opening.
I think Caro is good too but favor the French. Maybe not you but there are a lot of Caro players that openly hate French players

Caro-Kann has to be the most toxic
Whaaat?
They are so toxic against anyone that plays the French and it's very mainstream
I play the Caro, and I think the French is a very good opening.
I think Caro is good too but favor the French. Maybe not you but there are a lot of Caro players that openly hate French players
You could say the same for anyone.

not sure specifically, but KID players tend to really really hate losing, while Slav players accept defeat more easily

not sure specifically, but KID players tend to really really hate losing, while Slav players accept defeat more easily
I do play the Semi-Slav and I really hate it when I don't get a Botvinnik Semi-Slav. They will not play a good move but I still lose to it anyway but when they play an actual Botvinnik, I win most of the time.

About the London, I feel that, playing d4 and not follow in up with c4, is an insult to chess.
Look at this Steinitz game - World Champion! - d5 c5 - simple!

There's also a Caro-French thing, I so it in Gotham video (He's a Caro player) - when you can actually play the fun Nc6 and start pressuring (it's against the Advanced Variation only):
Minute 2:30
(I personally never had a problem with de french bishop and I think that the h7 b1 diagonal (-what a Caro Kann bishop is controlling) is pretty much empty - lots of squares there, but non of them looks important)

not sure specifically, but KID players tend to really really hate losing, while Slav players accept defeat more easily
I do play the Semi-Slav and I really hate it when I don't get a Botvinnik Semi-Slav. They will not play a good move but I still lose to it anyway but when they play an actual Botvinnik, I win most of the time.
White knows that people want a Botvinnik and that they're likely to know it above about 1700. Therefore, they play something else. For instance, how well do you know the play after
d4 d5, c4 e6, Nc3 c6, e4 de, Nxe4 Bb4+, Bd2 Qxd4, Bxb4 Qxe4+, Be2
That's an interesting, attacking line too, although taking on g2 with the Q is a bit dodgy for black, who should develop. Otherwise, white has any number of slower systems against the Semi-Slav which gain white a small advantage. For instance, the Meran. How well do you know that? Black has at least three major variations to choose from. Also, white can just develop and keep a small edge.
Interesting setup. I did face it twice winning one and drawing the other. But on Lichess, I'm 1900 blitz on Lichess which is about 1750 blitz on chess.com and people still don't play it. The only time I was able to play to the mainline was when I played Lefong but that was when I setup the position for him to play
Caro-Kann has to be the most toxic