To me, the most influential contribution in the last few decades would be "Capitalist realism: is there any alternative?"

Its very heavily influenced by Marx but it's more like a philosophical intervention where I would see das kapital as more of a historical intervention. As in "erm no, this is the story of how we got here" (das kapital). Although, there's plenty of philosophy of course. Its only about 5 pages long and there's free readings of it on YouTube.

Although, imo, the real origin story of capitalism covered in das kapital, on a national scale, is something everyone should be much more aware of.

What are you familiar with, what do you feel weak in? I'm a Marxist, so I can recommend some generally good introductory works, but if you have specific interests or pre-existing knowledge I can make specific recommendations.

Otherwise, I will always recommend the famous Yellow Parenti Speech.