I have posted this on the DS9 and VOYAGER side, but how about TNG? Same parameters... 3 appearances or more. And you can use a specific character as the reference.
Q (Someone mentioned them as a villain, and even though I don't count them as such, a case can be made having the Q being villains.)
Ferengi
Romulan
Borg
Cardassian
Q (Someone mentioned them as a villain, and even though I don't count them as such, a case can be made having the Q being villains.)
Ferengi
Romulan
Borg
Cardassian