Generally speaking, it is this sentence: "Proletarians of the world, unite!" (The last sentence of the declaration)

However, its opening sentence is also quite famous, "A ghost, a ghost of communism, wandering in Europe ..." (This sentence is a bit long, so I won't type it all).

It should be said, "Proletarians of the world, unite!" This slogan expresses the hope of great teachers Marx and Engels, hoping that the world proletariat can break the shackles of national boundaries, practice internationalism, unite against the bourgeoisie and other reactionaries, and strive for liberation. This is a very powerful sentence.