When the Klingons were less meat-headed I thought of them as on par with the Federation but ultimately a little weaker. Especially when you brought in the scientists.
The Romulans, by the 24th century, seemed to be a third superpower themselves but still less powerful than the Klingons.
The Cardassians at first were problematic but ultimately nonthreatening - the Galor Class Trager Macet commanded was dealt with easily enough. Later they seemed to be up there under the Romulans, but overall I see them as a nation the Federation would beat but at considerable cost - unlike the Klingons who they'd also beat but at a devastating cost.
Both the Romulans and the Klingons have the benefit of being much older nations which I think explains how they're still competitive with the Federation - they've had more time to conquer and invent. The Klingons also have the benefit of alien tech early on via the Hur'q. Still, given the UFP is already ahead of them, I see the gap only widening the further ahead in the future you go. Cooperation, science, and altruism are more efficient and competitive than force, threat, and anger.