Well, I do think the Cardassians are more interesting that the Klingons, but then again, I never found the Klingons that interesting. Sorry. Individual Klingons could be quite interesting, but en masse, not so much.
In TOS, they are plain old bad guys, and the fact that they sometimes have a sense of humor and intelligence doesn't change that -- lots of bad guys in lots of genres have senses of humor and intelligence, so that isn't enough to make them distinctive to me. In fact...they kind of remind me of some of the bad guys from film noire. Philip Marlowe interacted with guys a lot like that, and Sydney Greenstreet would have made a fine Klingon official, now that I think about it (aside from the fact that he was dead by then -- that would limit his effectiveness). Some of the bad guys in Clint Eastwood's spaghetti Westerns were kind of Klingonish, too.
In TNG, yeah, it's true, they pretty much become intergalactic Vikings. Which is OK, I still enjoyed many of the characters (as individuals) and story lines, but I just got tired of all the posturing and blood wine and testosterone (or the Klingon equivalent) and so on. But even there, they could be quite interesting. I think Gowron is a very interesting character, for example.
I think the Romulans are great adversaries, too -- they've always been my favorites -- but I don't think they ever realized their full potential, and I'm not sure the new Trek movie is going to do that. Maybe, though.
I guess I like moral ambiguities -- at least in fiction, if not so much in real life. And the Cardassians and the Romulans offer lots of that.