I had back problems from my crappy office chair that I used for more then a decade.

I searched for month for a good replacement, sat on every chair I found in nearly every furniture shop in town. After some month, I found the best chair I ever sat on, and it's by far, not the most expensive one ;)

http://www.officeline.se/products/hoganas-eco/

This one is the hit, you can unlock the base so you can use it in different heights. The backrest pressure you can adjust with a button, different locks and full flexible arm rests. I let everybody test sit on it who cares and I only heard excitement so far.

Combined with electric table I started to change my working position quite often and my back pain was gone after a month.