The butterfly-backed, fabric-and-metal Mirra 2 has loads of support and responsiveness, plus a killer feature for those with back issues: a pair of knobs on its spine that let you dial in the lower-back support. The chair makes switching positions smooth. The seat isn't as adjustable as others, however.
Dr. Aibinder says: "The wide, high back cradles the shoulders nicely – an important detail for overall support. And its structure is very supportive, without being too stiff. But the lumbar dials alone are worth the price of admission." [From $649; hermanmiller.com]