This will be controversial given the "that's not true communism according to my 170 year old theory book" crowd that sometimes permeates western leftists like a disgusting plague; but Xi Jinping. He's released 6 or so books, he has a literal Doctorate in Marxism, and has documented China's success and failures in its strive towards communism, including very good answers for every single criticism western Do-Nothing marxists have towards China's Dengist reforms and their incredible economic success.

I'm a fan of Cheng Enfu and Roland Boer. If you asked a few years ago I would have added Domenico Losurdo, but sadly he has passed.

A few that I can think of: Rockhill, Smith, Enfu, Boer, Manoel, Foster, Cockshott, Shaikh.

We lost Losurdo pretty recently, and he was IMO one of the best of the last few decades.

this is a tough one but I'm going to say karl marx

I'd say Michael Parenti

Lenin

Xi Jinping, Cheng Enfu, Wang Binglin, Tian Pengying, Xie Fusheng

Honestly it's such a broad area of research that naming a single greatest is impossible due to each researcher having their own speciality.

For real tho Annie McClanahan perhaps? I would have said Joshua Clover although sadly he just passed within the past year or so.

It's hard to sometimes decipher what scholar is or is not working with Marxism since it's been so influential to so many academic fields

not sure of "greatest living" but Kohei Saito seems decent

https://archive.org/download/18965473/Kohei%20Saito.%20Marx%20in%20the%20Anthropocene.pdf

I thought Karl Popper's insight's worth a read/look, but he was and then was not a fan (he dead though)

Surprised that no one's mentioned David Harvey, who's perhaps the best teacher for those new to Marxism

im adding Hardt and negri, and jodi dean.