The Expanse seemed to be pretty even throughout its six season run. Maybe the only real "ding" is the fourth season where most of Jame's crew is on a planet.
Firefly. I'm not sure if there was enough gas in that show's tank to have kept it on this list or if the plug getting pulled at 14 episodes did it a favor for us fan's collective opinion of it.
Though I'd imagine that "space opera" is easier to keep the story/plot quality pretty even as writers can always just switch to a different set of characters to avoid having to deal with writing their way out of plot corners.