"Albatross," no question. It's an interesting glimpse into McCoy's past, it sets up an effective dilemma with McCoy's arrest, it features an interesting alien race in the Draymians, and it doesn't get too silly in its sci-fi, beyond the astronomical error of auroras existing in space rather than being atmospheric phenomena. (But it could be rationalized if you assume there's a particularly dense interplanetary medium in the system, maybe an unusually active star that sends out lots of plasma and has an expansive magnetic field, or something. The "plague" could be some kind of radiation sickness.) It's a bit unusual for TAS in that it's a story that could've been told on TOS with little change beyond making the Draymians more humanoid. That makes it less imaginative than a lot of the episodes, but less silly than some.
"BEM" is fairly mediocre. The idea of a colony creature, a conscious entity consisting of several autonomous parts, is interesting, but not well-executed here; it's hard to believe such an entity could evolve such a humanoid body plan. And the way the parts just levitate through the air is silly. (I prefer the Foster novelization, where the torso walks on its hands and the head crawls on little cilia that come out of the neck.) And the whole "powerful alien entity judges humans until we convince it of our principles" angle has been done.