Here I suspect the answer differs based on whether one is talking about ops or FPs.

For FPs I would be led to think MM is the difficulty level that best represents the lore, whenever available, because bonus bosses are assumed beaten in the lore (most of them, on Wookiee, list a time and place of death). Some of them are available only in MM.

However, KDY and Colicoid Wargames don't have a MM, and don't have a SM either. Now, there are several FPs that don't have a SM but have a VM and MM, but these are typically among the oldest content.

For ops, on the other hand, I feel it's a toss-up between SM and HM, since not all ops have a NiM available, but EV is assumed run in 16-man in Wookiee because of the fourth boss, regardless of whether lore assumes it's run in SM or HM.