Mahatma Gandhi will always be remembered for his fight against untouchability. He was not against caste system. Because of him untouchabiility was officially banned. But even today untouchability does exist. Do you come across any tea stall in any village in India operated by a scheduled caste person?
Ambedkar will always be remembered for his contribution in securing reservations for SCs and STs.
Kanshi Ram will always be remembered for empowering SCs and STs. He made the people outside the fourfold society aware of their political rights. Creating awareness is a great achievement. In my view he is the greatest of the three.