Explanation: Karl Marx was a foundational writer in socialist theory, and effectively the father of modern sociology as well. His work is… not always riveting, as anyone who has attempted to tackle Das Kapital can tell you… but was extremely innovative and remains cutting and insightful (if imperfect) as to the nature of capitalism and its problems even today.

Lenin was the leader of the Soviet Union, the first lasting success of revolutionary socialism - though how successful (or socialist) one really wishes to count it is up for debate. Since we’re in tankiejerk, most of us will probably not count it as particularly socialist, but even nominal successes were novel. His interpretation of Marxism - even when it contradicted Marx - became dominant largely because he controlled the country-sized megaphone that was the USSR - a single-party state which largely followed Lenin’s dictates.

Stalin was the effective autocrat of the Soviet Union after Lenin’s death. His interpretation of Marx and Lenin - even when it contradicted them - became the standard for the next ~70 years of capitalist-socialist conflict, largely because he fucking shot every socialist who disagreed with him in the Soviet Union, and attempted to sabotage every socialist who disagreed with him outside of the Soviet Union.