I'd give overarching baddest of the bad to the Founders. Even though the Female Founder was the face we saw during most of the episodes, she was acting on behalf of the entire Great Link. They were xenophobic demagogues who had no trouble with the concept of selectively breeding entire sentient races to serve them. Both the Jem'Hadar and Vorta were evidence of that. And as though to add insult to injury, they don't even bother giving their "ambassadsors" a sense of aesthetics because it's not necessary for them to do their jobs. Fear that solids might try to destroy them is a piss poor excuse for what they became and the means they pursued to their ends.
Dukat was an overt malignant Narcissist. Every one of his behaviors and seeming contradictions can be explained within the context of that. Winn was a covert Narcissist. Ultimately, both of them were in it only for themselves. Any loyalties or beliefs they professed were utterly self-serving and about manipulating their own narrative to outsiders. Someone upthread said Dukat loved Kira. Not a snowball's chance in hell. He viewed her as an object to be won over for his own need to be adored and admired. He had zero interest in who she was as a person or how his pursuit of her would affect her. She represented all Bajorans to him, and he hated the fact that the Bajorans didn't see him as the great guy he wanted to pretend to be. Classic Narcissist BS straight from the playbook.
If they'd had the time and inclination to develop him more, I think Tain would have shown himself to be a contender.