Marxist-Leninist is the usual word for things — and people — that belong to Marxism-Leninism: the ideas of Marxism as Lenin explained them and added to them, including his belief in a tightly led party and in the “dictatorship of the proletariat” (government in the hands of industrial workers and poor farm workers, in that picture) as a step on the way to revolution. In real English this spelling is almost never the name of the theory itself. The theory is Marxism-Leninism. This word is the -ist form: a Marxist-Leninist party, a Marxist-Leninist state, a Marxist-Leninist (a person who holds those ideas). You will see it on the adjective side of a sentence far more than as a title on a book. Formal history and politics writing uses it; everyday chat in Taiwan is more likely to say 馬列 and not bother with the English shape. The hyphen matters here too: it tells you the two names are working as one label, not as “Marxist, and also Leninist, maybe separately.”