The one that seats you at a good height to work at, with your back supported, and is comfortable. Apart from that, it is up to to your preference. I use a typist's chair with castors, which means that I can push it back easily when I leave the sewing machine for the ironing-board.
I bought DH a leather chair from M&S for watching the tv on the computer on his desk. It is very comfortable with adjustable height and I now want the same for my sewing machine . I have a very good ladder back chair at the moment but the leather one is very nice.
I sit on a piano stool at the moment. It can be raised to the right height. I have used an orthopaedic saddle chair when I had back trouble. if you want a typists chair or office chair then visit IKEA - they have lots to try out on the spot. I find at the sewing machine that I bend forward and the back is not resting on the chair, height is more important.
I gave a good adjustable office chair to my step grandson when he went to uni, but wish I still had it because although I don't sew, I do now tie my own fishing flies and having a chair that is height adjustable and swivels would help me considerably. I use an old dining chair which is held together with glue and leather straps as it has collapsed on me more than once - usually when some tiny, intricate piece of work has gone pear shaped and I have thrown myself backwards in frustration and end up on the floor amid bits of chair!