Out of the races which are frequently named, but rarely seen, I would be inclined to say the Tholians and the Gorn.
I would also like to add those unmentioned aliens from ENT's "Silent Enemy". It's a shame we never saw them again, because there are too few non-humanoid races in Star Trek, and those for me intrigue me the most.
I would also like to add those unmentioned aliens from ENT's "Silent Enemy". It's a shame we never saw them again, because there are too few non-humanoid races in Star Trek, and those for me intrigue me the most.