The most caustic, for me, has been some video game communities. I refuse to have anything to do with MOBAs because the player-base is so frustratingly venomous and elitists drove me out of CS and Battlefield. I've seen some caustic subreddits but, on the whole, I wouldn't say Reddit is nearly as bad as some people think it is.
Since tumblr kicked porn off the platform, it has become interminably pointless. While some interesting things are still there, they are few and far between. I have no reason to go there if there's no delicious clop on it.
4chan is just a mutant stepchild at this point. They've long since become fucking worthless and outdated. It was once grand with clever edgy comedy and memes. But not anymore. I still visit /e/ on occasion though. Anime tid will always be wonderful.
However, the grand champion of shitty is Twitter, followed very closely by Facebook. Everyone and their mother can use these platforms to espouse whatever bullshit rhetoric they wish. Unfortunately, that also means extremism is highly concentrated in those places. I refuse to use twitter because it's the most venomous platform on the web. I do use Facebook but only to keep up with certain family members, and I'm part of PC big box collecting groups there too.
Reddit, tumblr and even YouTube comments aren't even in the same league as twitter.