This is only scratching the surface but I will present one of his most famous pieces to people who might ask why something like this is said. Keep in mind this was written 300 years ago. That's 300. fucking. years. ago. Think about how dated something from the 80s might sound. How modern does this sound? How completely universal is it's beauty? To me, this could have been written today and still sound fresh and beautiful.

https://www.youtube.com/watch?v=iWoI8vmE8bI

This piece is still deeply moving despite centuries of tastes changing. This is only barely scratching the surface of Bach. As a musician, when I listen to other great musicians speak, they all speak about Bach as the best. Of course that's subjective, and there are no 'wrong' answers on who is your favorite, but when the feeling is so nearly unanimous amount people who are often, frankly, contrarian and counter culture it says something.