Dremans and Bajorans.
One culture that I got the opportunity to explore, and another that I wanted to see explored more.
The others? Overexposed.
And you don't think the Bajorans haven't been over exposed?
As for a favourite alien species, I don't really have one, but I always thought both the Ferengi and Andorians were good, but that's because of Deep Space Nine and Enterprise respectively.