I believe many great human beings have existed throughout history, but the impact they cause is often limited by their circumstances. For example, there have been thinkers defending compassion towards human beings and animals in possibly every culture, but those sages live and die admired yet misunderstood. Their lives are seen as "extraordinary", and thus, not attainable for normal people like us. We give up on following them since the start, instead of trying to achieve their wisdom or understanding...

Anyway, by mere impact, I guess Socrates, Plato, and Immanuel Kant are on the top for me. The first ones were influential in the development of many branches of knowledge, and solidified a tradition of critical thinking since antiquity. Immanuel Kant is kind of recent, but I'd say his works were really important for discussions around philosophy, science, arts, religion, and more. I admire Immanuel Kant greatly. I was recently reading a little text he wrote about psychiatric disorders and he was predicting modern paradigms in the 18th century. He was such a brilliant and knowledgeable person.

There are also incredible inventions and discoveries that have helped us all, but often those are the results of collective efforts. Still, as I said before, amazing human beings the ones that gave and still give these things for free. Getting personal again, I wouldn't be alive without many of those advances (vaccines, medications, etc.). On the technological side, the founder of that website that unlocks academic papers has had an impact that is yet to be analyzed.

Sorry! So many people...!