A counterpoint I read on HN back in April, wherein the author decries expensive office chairs, and recommends exercise balls and tub chairs: https://news.ycombinator.com/item?id=26919449
Exercise balls (or ball/chairs to avoid rolling), or ottomans for me. You can sit with your hips rolled forward / legs angled down, feet under you, and spinal column balanced above your hips. Comfortable and ergonomic.
I read "tub chair" and thought of Dalton Trumbo in his bath office. I'm sure it's not ergonomic, but I bet it's comfortable and relaxing.