Was looking around as well, so just to share some thoughts. Your millage may vary as the extent of ergonomic-al comfort varies from individual to individual. You can Google for these chairs for more info, and suggest that you go and try them personally.
1) Herman Miller Aeron or the newer Embody: Many people choose the Aeron, but to me, the Embody wins hands down for its comfort. Didn't like the upper backrest curvature of the Aeron. Only downside is that HM chairs don't have a headrest. Embody's the most expensive of the lot at ~$2,500.
2) Humanscale Freedom: Pretty comfortable, no frills chair (philosophy of Humanscale is that there are no knobs/levers), but the quirk is that when you recline, the headrest pushes forward to maintain proper viewing angles to the screen. It's a feature, but didn't really like it.
3) Ergohuman v1: Singapore didn't have the v2 chair. I bought this in the end as it fits my requirements of a headrest, and comfort levels were reasonable. Cheapest chair of the lot too.
If money's not an issue and if you don't mind having no headrest, get the Embody. There's also an MO for Aeron at HWZ as well. Good luck on the search!
