Ultimately, a chair is a personal choice best made after sitting in one for a few hours. Every body is different. Lacking this, I would look to ergonomics. The Swedes are famous for their ergonomic furniture. Many of the designs listed by @Tropical_Willie above originated in Sweden.
Sciatica is when the sciatic nerve that leads from the in the ischias region of the spine to down the back of the legs has been damaged, most often by impact or by being pinched between vertebrae. It takes a long time to heal and healing requires that the nerve not be further traumatised by pressure. The pressure of merely sitting or standing upright, placing weight upon the lower vertebrae can keep the nerve from repairing itself and the pain can be lifelong.
You’ll notice that all the “conventional” chair designs have lumbar support. This helps relieve the pressure on the sciatic nerve.
Many of the “unconventional” ergonomic designs shown in @Tropical_Willie‘s link do just that by displacing a lot of the pressure normally placed on the sciatic nerve to the knee support in the design. This design displaces pressure more efficiently that the lumbar support. This chair was initially meant to be used intermittently throughout the day along with a more conventional design.
The knee-support design.
Here is a more conventional design. Notice the lumbar support, head rest and pay special attention to the divided seat. This design forces a portion of the pressure that is normally placed on the sciatic nerve to the buttocks, thus giving some relief.
I recommend using both throughout the day. I also recommend you look at the IKEA site first for these designs. IKEA has very high standards of production and consumer protection rules and are able to sell these original designs at affordable prices due to mass production. These designs used to be extremely expensive due to limited Swedish production, but IKEA changed all that.
@janbb I know of an excellent 20-minute, very passive exersize you can do twice a day at home or the office that will do wonders for your sciatica if you are interested. It provides enough rest for the nerve to begin to heal—which once the process is allowed to begin, can take nine-months to a year.