Gonna respond to the one's I know well enough to make a semi-informed contribution to.
Aku (The Master of Darkness is not to be trifled with child!)
The easy example, honestly. An array of different abilities and undefined powers; likely to play with his food a bit but even still, a head on fight will have you dead. Possibly a vulnerability to super holy weapons?
Acts as an antithesis to Fathers own powers, so I imagine the end conclusion is based on who can overpower who with their heat or cold.
Makes sense, but probably less dangerous than the above two; Vilgax is a genocidal warrior and has bested all sorts of aliens, but he's still limited in essentially being a brute and a capable/competent leader. Could he punch out father and knock his head clean off? Probably yeah, but he's limited in not having powers.
Unless he has the Omnitrix - that changes things.
These one's I know. So they've got a pretty inclusive Superman style powerset, from ice breath to laser vision; they match feats with the PPGs, mirroring the girls, who themselves can hear all the way from the moon to the Earth (don't ask me how...), they can also traverse to the moon and back to the Earth in maybe... I might be misremembering, but it's near instantaneous from what I recall.
Given the RRB are consistently their closest physical contender, sometimes outpowering them, I imagine any of the three is one of the really serious threats.
However, two out of the three aren't really very intelligent, and Brick (albeit when he was a child) was himself not the sharpest tool either. So that depends on how the three developed.
Which makes me think - acquiring Chemical X could be a worthwhile venture.
Less overt than the rest, and that makes her dangerous. Certainly more blunt than her uncle, but manipulative enough to makeup for her lack of sheer knowledge; girl has potential and skill. Or she did, since however many years ago Ben 10 took place.
