The Tholians. Thoroughly alien, enigmatic, vaguely arthropod-like (detectable even in TOS, I felt), and totally xenophobic. None of this "gosh, we're out in space, let's make friends with folks" stuff for them.
. Punctual, too.
It's also interesting too see a non-humanoid species having an organized society technologically developed.
The TOS Romulan are also pretty interresting as a potential "lost tribe" of Vulcan.