I find that the best ergonomic chairs are the ones that just fit you, with zero adjustments and knobs and sliders. You have to go into a store and sit in them to find that out. Personally, this one is good for me:
If I hadn't sat in it myself, I wouldn't have believed it. (It's the surprisingly expensive Polo 5-Star, a reproduction of a "classic" design by some famous U.K. furniture designer.)
If you need a full-backed chair, the really cheap "executive" chairs are good candidates, but again, you need to sit in a variety of them to find the right one.