It really depends, for short but powerful quotes Marx is the king but for longer ones i really like this one by Engels, it makes me hopeful:

“The splitting up of society into a small class, immoderately rich, and a large class of wage-labourers devoid of all property, brings it about that this society smothers in its own superfluidity, while the great majority of its members are scarcely, or not at all, protected from extreme want. This condition becomes every day more absurd and more unnecessary. It must be gotten rid of; it can be gotten rid of. A new social order is possible, in which the class differences of today will have disappeared, and in which – perhaps after a short transition period, which, though somewhat deficient in other respects, will in any case be very useful morally – there will be the means of life, of the enjoyment of life, and of the development and activity of all bodily and mental faculties, through the systematic use and further development of the enormous productive powers of society, which exists with us even now, with equal obligation upon all to work.” – Engels, Introduction to Wage Labour and Capital.

As for Marx quotes, i like this one putting emphasis on the last punch:

“Men make their own history, but they do not make it just as they please; they do not make it under circumstances chosen by themselves, but under circumstances directly encountered, given and transmitted from the past. The tradition of all the dead generations weighs like a nightmare on the brain of the living.” - Karl Marx, The Eighteenth Brumaire of Louis Bonaparte.

Also not Marxist, reactionary actually, but i wear this one with pride:

“With dialectics, the rabble gets on top.” - Friedich Nietzsche