I have the Ergohuman. It's perfect, I can sit here all day without any aches or pains.
https://youtube.com/watch?v=MwGN-Uu9fpg
It cost me more to ship it here from the US than it did for the chair itself.
I was going to go for the famous Aeron chair but I wanted one with a headrest.
You might also want to look at the fluid chair.