None of the above. Correct answer: Species 8472. Huge new utterly badass enemy in their first appearance... followed by being neutered shapeshifting puppies that just really want to understand us so we can be friends in the very next appearance. Biggest waste of a bad guy since Darth Maul.
While I agree they were wasted, I can't say I feel they belong on this conversation. With Species 8472 it felt more to me that they were limited by the budgetary or studio demands placed on the show rather than what I saw on screen (until the "In the Flesh" episode which did pretty much defang them and turn them into Changeling rip offs). Up until that point though, they were pretty scary, but the show failed them. Out the gate the Kazon and Ferengi were lacking in a way that Species 8472 were not. Species 8472 got a great introduction but the follow-up wasn't strong. I felt with the Kazon they got a weak introduction and never fully recovered, and did need Seska to make them formidable. The Ferengi rallied after becoming more comedic, though still retaining some sketchiness.