I tested many of the anti-fatigue mats, but personally prefer the "Imprint CumulusPro Commercial Grade". It's heaven on your feet, and it makes a huge difference once you start standing 6+ hours a day.
Unfortunately, the CumulusPro is out of stock everywhere, but I found you could order it online at a significant discount on Costco ($69 vs $115). Here's the link.
My next thought was to look into kitchen mats as line cooks tend to stand for long periods of time.