Of the big ones:

sh.itjust.works - biggest comms are a pro-NATO circlejerk comm and an anti-communist drama comm. Its users are a mixed bag due to it being a "generalist" instance but many are straight up fascist, the classic Nazi bar.

Lemmy.world - Reddit but more acutely Reddit than Reddit itself, because those that dislike Reddit don't sign up for it but instead choose other instances. Overwhelmingly liberal, mods and admins lean Zionist.

dbzer0 - Excellent piracy comms. Generally an anarchist instance, generally anti-ML, but federates with Hexbear.

Hexbear.net - One of the two genuinely great instances, alongside Lemmygrad. Left-unity, so sectarianism is bannable, but majority ML and explicitly pro-AES. Has anarchists, Maoists, etc as long as they don't complain about AES. Has a mostly good News megathread, and is far more casual than Lemmygrad.

Mander.xyz - Science based instance. Some of the mods and admins are pretty cool, too, not generally anti-communist like the liberal instances.

Lemmy.ml - ML mods and admins, mixed bag users. Widely federated, so a constant stream of arguments. Best spot for agitprop on Lemmy IMO if that's your thing.

Blahaj.zone - queer-focused instance for people that oppose AES. Liberal, but fights with Lemmy.world and sh.itjust.works for them sometimes being anti-LGBTQIA+. The explicitly leftist queer instance is Hexbear, so the rest tend to go to Blahaj.

Lemm.ee - dead

Lemmy.zip - Mixed bag, some fash some leftists some liberals. Truly a random grab.

That's all I can think of! In general, Grad is one of the most defederated instances, followed by Hexbear. The 3 Marxist-friendly instances are generally on good terms, and the rest of Lemmy constantly saber-rattles against us.

I have accounts on Hex, Grad, and .ml, and use all 3 depending on my mood. I'm technically most active on .ml but I enioy my time on Hex and Grad way more.