Least likable alien species
Which alien species was the least likable one to you?
For me, it was the Qomar, from the episode "Virtuoso" (the one where the Doctor introduces the concept of music to a culture that only cares for mathematics). They were thoroughly annoying, two-dimensional, incapable of empathy and probably the most arrogant people in all of Star Trek.
The outcome of the episode wasn't really a surprise - a painful lesson for the Doctor which he should have seen coming from a mile away (and everyone else did). It seems the writers decided to make him temporarily stupid. Or he was just blinded by his own arrogance, of which he has plenty (and HE can pull it off, because he's a great character played by an awesome actor, and he's very much capable of empathy).
If you're really looking for arrogance though, go for the Cardassians. They're the undisputed masters.