That is a very good question, there are lots of great and important ones. I think personally Kopernikus, as he changed the way we looked at our planet and the sun. Da Vinci because he was amazing in so many ways, Marie Curie cause she worked very hard and paid a tough prize for her research and yes, Einstein because so many things changed after him and he is so crucial for lots of discoveries today
I wouldn’t say that there is a single greatest scientist better than anyone else. I would say that there are some scientists that managed to spot something extremely obvious yet unnoticed and they managed to explain it in a simple and function way. That said, everyone has his own preferences and role models. My favourite scientists are Alan Turing, Charles Darwin and John Nash and they influenced me a lot among the classics. On top of inventing computer science, Turing asked some questions on AI and intelligence still unanswered and actual today. Nash defined formally how two selfish people can came to an agreement and his work had implications on a branch of AI (called computational game theory). He also inspired to study the opposite problem: how you force two selfish people to stick to fair an equal rules (this is called Mechanism Design and the inventors of this theory got the Nobel prize some years ago), this theory ended up in my bachelor thesis and aided me in taking my future career decisions. Darwin’s studies on the theory of evolution paved the way for one of my scientific results that demonstrate that the grasping skills of robot hands might be limited because the opposing thumb is not correctly represented and it is not just a hand control problem. I found a confirmation of this by studying the findings of evolutionary biologists which branched off his original ideas. If you think that all those three people and their ideas were very bullied in their last years this adds up to my respect for them.
That’s a hard one! So many scientists have had huge impact in what we know today. If I had to pick only one, I would pick Newton. He invented so many fundamental parts of mathematics and physics and at such a young age. He was incredible!