It is my unprofessional opinion that riders over 200lbs should be on a horse with a very thick, rugged build. This brings to mind Percherons, Belgians, draft crosses, etc. The key is to make sure that the animal's LEGS and BACK can support the weight.
I think that the opinion that drafts are not the best suited might stem from the same reason that minis are not suited for riding: they are built to PULL, not carry. However, modern drafts and especially draft crosses are built for riding more so than pulling. The key is to make sure the horse's legs and back can support you.
So in conclusion, I would suggest looking for well-developed, middle-aged (10-18 years) draft crosses. I popular place to start might be a Percheron/QH cross, or Belgian (the "American Belgian" is actually technically the Flemish Horse) crosses. Stay clear of such crosses like Georgian Grandes (Saddlebred x Friesian), as beautiful as they are, they are still lighter than necessary.
You might find a Draft Mule to be very suitable as well - smart, big boned, and capable of being very nice all-around mounts, Mules are great trail companions as well; they are steadfast and surefooted, and often times are prone to protect the group in the event of danger.
Hope this helped some!
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