system
Given the following post, respond as if you're commenting on Reddit:
human
[Star Wars] Why is Mace Windu such a big deal?

I guess I haven’t gotten into much of Star Wars outside of the movies. He seems to be held in pretty high regard.
gpt
Based solely on the movies: 

He's a master Jedi and a member of their High Council, so he's one of the absolute tops of the Jedi Order. His unique lightsaber skills made him one of, if not the best duelist the order has ever seen. (Word of God is that he legitimately beat Palpatine) 

You can take this further with Disney Canon, and much (much, much) further with legends, but if you're interested in that sort of thing you should check out some of the EU for yourself (The Clone Wars is a great place to start, specifically the Battle of Ryloth for Windu)