GC is what I've been using for years, and it's usually extremely close, but I've run into a few occasions where it was off by a small bit, which I noticed because, e.g., it told me a route was just under 22k, but when I went to book it as a xCIR22, the AA RTW desk said it was a few miles over 22k so I had to adjust the route.

It's fun to see the routes on a map as well as the miles.