I really do think Abraham Maslow was one of the most "ground breaking" psychologists. There are plenty of reasons why I think so, So I will just go through all of them!
First off, the school of though he belongs to is the Humanistic school of though, which is one of the best in my opinion. The reason for this is because they finally decided that people do in fact have control and free will over themselves instead of being slaves to their own conscious. To top that off he was the founder of this school of thought in the first place.
He lived in the 1900s, Mid to late is when most of his life in psychology happened. Before he started studying psychology he actually went to school (City College of New York) for law, but switched over to psychology shortly after. He liked to study off of other psychologists writings a lot. He was influenced by Gestalt psychologist Max Wertheimer and anthropologist Ruth Benedict and was a huge fan of them.
Around the time when he was alive a lot of wars and assassinations where going on. Also in the beginning of the 1900s The Great Depression was happening. This could have lead to a lot of advancements because of the Humanistic psychologists, Maslow in particular. He developed a Hierarchy of needs named after himself, setting up the needs of human life for psychical and mental health. This could be linked to people struggling with money or hurtng because of things happening due to all of the wars. These ideas could have also been used to help people with those issues at the time. They also decided to start taking a more friendly and personal approach to their patients, this could also be be spawned from people going through really rough times and needing people to just be there for them and help them through all of the negative turmoil going on at the time.
Maslow was a very important man as far as developments in psychology goes for many reasons as I have said before. In a time where all psychologists focused on what made people abnormal, Maslow tried to focus on the positive side of it all. He had a huge interest in human potential, and helping people reach their mental peak of health, and that left a huge impact on further developments of psychology for the better. He also began to work more personally with people which proves to this day to work extremely well and so much better than previous methods of psychiatry. His friendly and "I am here to help" ideas for psychiatry helps a lot and gets a lot further with patients and they still use it now a days. Maslow also, as I stated before, developed a Hierarchy of needs. The pyramid sets up a clear depiction of the levels of needs people have, leading all the way up to self-actualization which would be the peak of one's mental health. This made it easier for people, and psychologists alike to understand where they are at and why, also how far they need to go to reach a certain point in their mental health.
Maslow was a very important man to psychology, and he was most defiantly a "Ground Breaker" for the entire movement of psychology.