It never occurred to me to search for "best." I just pick all edibles and mix them up in the tortoises' diets. Because I offer a wide variety, I don't key into just one item as being 'best.'
I agree with Yvonne.
To say that one was the "best" would imply that it provided complete and perfect nutritional value.
I can recommend some good options.
Hibiscus leaves and flowers
Rose leaves and flowers
Grape leaves
Hosta leaves
Dandelion leaves and flowers
Cactus pads (Opuntia/nopales)
Aloe vera
The list is long and varied..
Have you seen this website, it can be helpful. http://www.thetortoisetable.org.uk/index.php#.WoxX_OCIY0M
If something is "best" it would be cactus pads. I think... at least that is what I understood. Anyone can correct me if I am wrong. But variety is truly best, like said.