Well their are lots of factors that can effect the comfort of a bit for your horse.
Your discipline, if you want to compete, your skill level as a rider, how your horse is trained and the shape of your horses mouth. I use a hollow mouthed loose ring with the horse I ride. It being hollow makes it very light for the horse.
I really like french links as well. It really depends on what the horse likes too, some horses do well in a loose ring snaffle and some hate them, etc.
Also, a more mild bit is one that is thicker because the pressure is less concentrated and more even distributed if that makes sense!
Eggbutt french link is my coming 3-year-old's fave. He hated the copper mouth loose ring single jointed snaffle I originally had him in. He was okay in the loose ring french link. Now we have an eggbutt french link and he's wonderful (when he's not deciding that he is, indeed, three and MUCH bigger than me lol).
I think the mildest one is eggbutt oval mouth (or french link, but both my mares preferred oval one) made of copper, sweet iron, cupreon, and alike (something with taste in it). With this being said the preference varies from horse to horse: my qh prefers nice, stable eggbutt, however my paint likes the moving O-rings. Also while thicker bit is considered more mild, some horses have small mouth, so much happier with the thinner bit. And so on...
So the bottom line you won't know what your horse prefers until you try different ones. :)