Adjustability, support, and comfort. This helps keep the spine in alignment and relieves the stress on the back and neck. Many chairs have adjustable arms that allow you to have them adjusted to the height that is most comfortable for you. That height should be so that when you rest your arms, on the chair's arms, your shoulders are level and square. Lumbar support, at the right level for you, is important.
Some chairs have adjustable lumbar support that can be moved up and down, and in and out.
Just be sure that the chair you use offers the support you need. Comfort is an individual concept. I like lightly padded back and seat cushions which conform to my body and hold it securely. I like to add an additional soft cushion which provides warmth and massage via nodules ,magnets, and a fluffy wool cover. Most office supply stores offer a variety of chairs at a variety of prices.