It's worth going to a Herman Miller dealer to try the range of models. Some of them fit different body sizes, especially varying heights and widths much better. I didn't like the way the Aeron felt on my shoulder blades, it's very wide, so I got the Embody which is also quite popular, and much more narrow. The Embody is more expensive but worth it in my opinion. Also remember that both come with an incredible warranty.

The only models I wouldn't spend much time looking at are their conference room chairs -- the ones not designed to be sat in comfortably for a full day.

More info on Aeron vs Embody - https://www.btod.com/blog/herman-miller-aeron-vs-embody/

The Aeron is the tried and true classic. The Cosm is an 'updated' version with different back heights, and The Mira is an easier to clean plastic back version. I swear by my Aeron that I picked up at a used office supply store for $400 along with a new desk.