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. :)
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