The essence of science is experiment and falsifiability, not grand philosophical theorizing. Grand theoretical thought has its place in science, but most often in order to design experiments, to create falsifiable hypotheses.
Libertarian socialism is a true "scientific" socialism, not in terms of its theoretical basis, but in its practice - by calling for spontaneous cooperation and organisation along with radical decentralisation, it allows myriad experiments - a practical science of social organisation. By overturning the sterile uniformity of the state, an anarchistic society could see areas alongside one another using different models of social and economic organisation - mutualism, syndicalism, communism.
Possibilities for experimentation abound: communal versus individualist settlements; highly automated industrial regions versus agrarian eco-communes and religious societies; decisions by consensus versus democratic voting versus networks of small, non-institutional affinity groups.
If you believe Marxists, there has never been a society "truly" organised on Marxist lines - the USSR and other "communist" societies are widely accepted to have been nothing of the sort. On the other hand, we have clear instances of libertarian-leftism in action - Maknovista Ukraine, anarchist Catalonia (read Dolgoff's The Anarchist Collectives), cooperative economic institutions, factory occupation and self-management movements, small Intentional Communities (communes), the student movements of 1968, Occupy Wall Street and so on. Thousands of written accounts exist documenting the successes and failures of these movements and institutions.
So, do you agree that libertarian socialism is the only true scientific socialism?