Well, there's quite a lot in the Marxist tradition that I think is valuable. For starters, Marx and Engels themselves are still pretty insightful. The Paris Commune forced Marx to reconsider a lot of his previous statism culminating in Civil War in France which describes how a dictatorship of the proletariat could look like, i.e. something radically different and bottom-up from what the later Bolshevik state apparatus eventually looked like. Critic of the Gotha Program is also highly crucial as it develops Marx's conceptions of what lower and higher phase communism could look like, implying that even lower phase communism is already stateless. Engels also has some later revisions like in one place he says that the communist demand for centralization became untenable when it came to light that decentral polities represented working class interests over the central polity. Of interest is also Bakunin's parallel discussions on the Paris Commune and his own critiques of the Gotha Program.
Second International Marxism can also be of interest, but the general perspective towards national chauvinism and revisionism largely weakens the enterprise of Second International Marxism. The strongest currents in the Second International were its left wing led by the likes of Lenin, Kautsky, Luxemburg, and Pannekoek. If you can move past some of the weaker works on the state, there's a lot of value there. Even reading the works on the state, like Lenin's State and Revolution and Kautsky's work on the state can help you develop your own perspective on why you disagree on their positions.
The collapse of the Second International due to national chauvinism in the First World War led to the systematization of new tendencies, namely Bolshevism and Comintern Marxism. This is really where Lenin, Kautsky, Luxemburg, Pannekoek, Bukharin, and now also Bordiga and Gramsci shine. Various debates in the Comintern developed new lines of development on the questions of participation in trade unions, in parties and parliaments, and in questions about valorization and commodity production. Even reading Stalin and Trostky is of interest so you can understand their position and figure out on why they think that and whether you disagree with them.
Bolshevism was largely liquidated by Stalin in favor of a new systematization in what is now known as Marxism-Leninism. Generally, I find very little value in Marxism-Leninism as it was merely the justification for the unconditional defense of the regime in the Soviet Union, Trotskyism has some interesting critiques of the bureaucracy, but largely Trotsky would have been just as brutal a strongman as Stalin, given Trotsky's earlier reputation in Kronstadt.
Left communism is of more interest to me. The Dutch-German left communism, also known as council communism, developed a libertarian communist perspective while keeping within a Marxist tradition. The Italian left communist tradition (often erroneously known as "Bordigism" after Bordiga) is also of interest in their critiques of capitalism in Russia and of liberal democracy. I'll leave it up to you to figure out what you think of their perspective on organic centralism, vanguardism, and the proletarian semi-state.
Some Trotskyists had libertarian flavors even while hating anarchism, like Draper. Post-Trotskyism like Dunayevskaya's Marxism-Humanism and C. L. R. James's work has libertarian undertones, especially with their critique of state capitalism in the Soviet Union and forwarding new articulations of humanism and socialist democracy. Posadism is a bit of a bastard child of Trotskyism, due to the whole UFOlogy, nuclear war accelerationism, and dolphin stuff, but Posadas still developed interesting work in technology and socialist-futurism. Another post-Trotskyist was Guérin who wanted to synthesize Marxism and anarchism and develop libertarian socialism. Bookchin could probably also be considered a post-Trotskyist as he did carry over his Trotskyist predispositions on the bureaucracy in his anarchist period. There are probably a lot of other Trotskyist tendencies that are worth looking at, but these are at the top of my head.
Western Marxism also has quite a number of interesting figures. Lukács looked into reification. I have been meaning to engage with his work more, but haven't yet, but his work will be important for working on reification and class consciousness. Lefebvre is also particularly interesting because he developed a "right to the city," arguing that just like workers make value, citizens make the city in their everyday actions, and that just as workers are entitled to the full value of their work, citizens have right to the city and all that entails.
While I am dismissive of Marxism-Leninism, I have a more positive opinion on Maoism. Mao himself synthesized Marxism with Eastern philosophies, particularly so with his notion of dualism in contradictions. While Mao was a supporter of Stalin, his critiques of the Soviet Union has some value. Mao is interesting because he truly believed in an almost anarchistic faith of empowering the people to emancipate themselves. Maoist China was capable of truly historic levels of mobilizations that was unprecedented before it and no mobilization after him can compare to the truly massive scale of social mobilization. His greatest mobilization was the Great Proletarian Cultural Revolution, partly a scheme to get rid of his rivals in the Party and also an authentic desire to empower and mobilize society. Mao is also contradictory, as he suppressed the most radical wings of the Cultural Revolution and put his own project down the path of capitalist restoration.
The most important Maoists outside of China were I think the Black Panther Party. Ideologues of the Black Panther Party fused Marxism and Maoism with the Black radical tradition to create a specific ideological program in the imperial core. Unfortunately, the Black Panther Party was beset with authoritarianism and misogyny, which brought its ruin. Black anarchism and Black anarchic radicalism (BAR) were developed in both to reclaim and defend the radical heritage of the Black Panther Party while rejecting the authoritarianism and misogyny.
There are, of course, other Maoists like those in the Philippines, Nepal, India, and perhaps the eradicated Indonesian PKI (which took points from MLMZT instead of Maoism), but generally I find their theories to be quite vulgar. However, they're clearly doing something right as all were able to mobilize significant portions of society for a protracted people's war. There are some Maoist thinkers that I respect, like Moufawad-Paul, Deng-Yuan, Pao-yu whose writing are all intellectually stimulating even if one disagrees with it. Unlike many Maoists I've met, they are not intellectually dishonest. Other Maoist parties have also develop criticisms of the People's Republic of China and other so-called socialist states, calling them social imperialist and/or revisionist, which are useful I think. I would not suggest Sison as I feel his writing is highly dishonest and vulgar.
For the de-Stalinized Marxism-Leninism, you could look into Lagman whose critique of Sison brings Philippine Marxism-Leninism back to Marxist and Leninist fundamentals and reworks the Rejectionist tradition back into the Marxist fold. While his Counter-Theses are in English, unfortunately his Theses are in Tagalog, so his reconstructive program for Marxism-Leninism has yet to be translated. Davis is also interesting because her de-Stalinization brought her to develop abolition democracy as a framework. It is still Marxist to an extent, but less explicitly so.