it's interesting that the Klingons, which are probably(along with the Vulcans) the most recognizable and famous of the Trek species, are not getting much of the vote here.
As I wrote, I find them one-note and boring, but I thought they were pretty popular among fans.
I think the Klingons could have been fascinating if they had taken their culture and such in the direction written by John M. Ford in
The Final Reflection. He gave them a unique, alien culture and outlook. As they were defined by TNG as simply a viking/samurai cross pollination, they were boring.