Herman Miller Aeron, expensive but worth it. About a grand new, but there are plenty available refurbished. Prices have gone up since I got mine - I paid £300, but they seem to be starting at around the £450 mark now)
Herman Miller Aeron or Mirra seem to work well for most people. Other brands suit certain people. Human scale is one often recommended. I have switched from HM to a Hag Tribute which I much prefer (but the HM mirra was very good).