Most basically I'd recommend Marx, but maybe not without some guide to what it is exactly you're reading, why things are peculiar and the tradition which Marx inherited, which as his collaborator Engels admitted, was moribund even by the time Marx published
Capital[0]. Although you can't avoid overlaying some interpretation, I'd recommend reading
Capital alongside something like Johan Fornas' "Capitalism" (Routledge, 2014) who tries to mention various interpretative traditions, and for something more opinionated, Michael Heinrich's "An Introduction to the Three Volumes of Karl Marx's Capital". Value is a big topic in Marx, and while neoclassical economics dismisses it (what I believe to be out of hand) I.I. Rubin, who was persecuted by the USSR under false accusations, wrote a
fantastic set of essays on his theory of value, named just as a collection of I.I. Rubin's essays.
Were you asking about philosophy in general? In that case, I admit I'm not very well read at all, even on Marx's Hegelian background - but there is a rather famous 4chan /lit/ board guide to philosophy from the start, though in my experience, it need not be read sequentially.
By philology I was referring to the study of the interpretation of Marx specifically - in my opinion it has produced some fascinating results (see Heinrich above, and importantly The Mismeasure of Wealth, which is a a set of essays by contemporary Marxian philosopher Patrick Murray[0]).
Also of interest is the discussion on Marx's more qualitative points besides his theory of value, specifically his theory of history, alienation, technology etc. for which Gerry Cohen and Sean Sayers have plenty, and Peter Hudis[2] claims from a philological point of view, it is easy to see how the USSR's style of socialism would probably come under fire from Marx himself.
On the whole, I'd just like to conclude with the fact that Marxism is very much not a unified school of thought, and in my modest and relatively uninformed opinion, it has only increased fracturing and has been marred by unfortunate internal hostilities after the 70s in which various philosophers tried to salvage what remained, reinvent, reinterpret or deny the grounds on which famous economists like Paul Samuelson and Piero Sraffa dismissed him. Marxism is so fractured that specifically regional schools have emerged, for instance the Uno School in Japan, AM in the Anglo-Saxon world, the Frankfurt School in 20th c. America, Neue Marx-Lektur in the German world etc.
Despite what it may seem, you asked for what I found worthwhile, but I've replied by citing almost all the authors who I've read at least one work of - I've read so little that I literally can't select what's worthwhile and what's not. This means I'm providing a limited picture, so take what I've said with a Capital-sized grain of salt!
[0] The "dialectical presentation" has come under fire from some Marxists as being insufficiently rigorous, outdated etc. - this is the Analytical Marxist school, which Roberto Veneziani (who you might also refer to on the matter of mathematical Marxism, along with Kliman's easy (but thick) "Reclaiming Marx's Capital") has written about: https://papers.ssrn.com/sol3/papers.cfm?abstract_id=2125671
[1] "Probing Marx's dialectical accounts of the commodity, value, money, surplus value, wage labour and capital, The Mismeasure of Wealth establishes Marx's singular relevance for critical social theory today."
[2] Peter Hudis - Marx's concept of the Alternative to Capitalism