It's funny how subjective this is. I have tried out most of the chairs mentioned in this thread and in general find the Herman Miller ones to be the most comfortable. My absolute favorite, which I have at home, is the Mirra. I think its lumbar support is simply amazing. In comparison, the Aeron is pretty good but I just didn't like it as much. The Sayl is also very comfortable and I liked the top-of-the-line Embody the least.
My next favorite is the Steelcase Think, which I like a lot. My opinion on the Leap is it's just okay, but I don't love it. I have a Leap at work and find myself adjusting it all the time because I'm never quite comfortable.
On the other hand, I found the Humanscale chairs, Freedom and Liberty, uncomfortable to the point where I didn't want to even try it for more than a few seconds. They recline in a very specific way, which I am really not a fan of.
The Knoll Life is hands-down the most uncomfortable chair I've ever sat in, and by that I mean I would rather sit in a cheap folding chair from the discount store. The Leap at work replaced the Life because it was literally the first chair that gave me excruciating back pain. Our office is full of them and many people find them uncomfortable. To quote a co-worker, the lumbar support feels like someone digging an elbow into your back. Removing the lumbar, I find the shape of its mesh back to promote very bad posture that would also lead to discomfort and pain.
