the cardassians are my favorite trek villains. they're complex and interesting, a strange combination of dignity and barbarism, without seeming too similar to the more simplistic and borderline silly "warrior culture" of the klingons. two of my favorite characters on ds9 were cardassians. the fact that it's always sort of unclear whether they're "villains" per se is what makes them so interesting and fun to me. the borg are boring because, as mentioned before, they're simple: big hordes of mindless space robot zombies. that's cool, but it's limited as hell.
plus, they look like they have dragon necks or something. the design of them is just really great, from the concept to the way they look physically.