The Mirra, Aeron, and Embody are in the top 10 for ergonomics. I've been in the Mirra and Aeron and prefer the Aeron, but some people prefer the Mirra. Either way, you owe it to your neck, back, hips, shoulders, and knees to get a chair that fits right and gives you proper support if you're going to be at a desk for 9 hours a day if you can afford it. Luckily, there are tons of reputable discount office liquidators online and on craigslist that sell these chairs at a 10-30% discount.
I believe that Staples, OfficeMax, and Office Depot have a bootleg version of the Aeron, but the fabric is different and isn't as supportive.