How many characters do you know on Nirn(past to present) that could rival a daedric prince? in terms of intelligences or combat?

The tribunalllllll

To be serious, Vivec probably technically came the closest. Seriousness aside, I'd go with Gaenor for combat ability and the Adoring Fan for being able to come back to life only three days after being killed, which is way faster than what it took Dagon to "awaken" from his banishment.

Tiber Septim or Vivec I would guess.

Mannimarco?

Not many, I think. The princes seem to wield the sort of power that typically makes physical combat unnecessary, and their actual spiritual/magical abilities are blessedly ambiguous.

An avatar of any of the Aedra would presumably be a match for them. The Tribunal at the height of their power. Vehk certainly proved what can be achieved with poetry, perseverance, and a little divine meddling. Aless could totally beat up Dagon really, really hard if she wanted to.

The prisoner can potentially make mincemeat of all of them, despite (or because of) him/her being a big ?

Any mortal that became a god.

Tiber-Wulfhart-Arctus as Talos, Vivec [censored] Azura in front of everyone at his trial then banished her, Alamexia and Sotha Sil took on Mehrunes Dagon and won (barely, but that's probably due to Alamexia being an idiot), the Battlespire Cadet defeated Mehrunes Dagon and his entire army IN [censored] OBLIVION! And this guy/gal was only a [censored] green Imperial Battlemage! That is how hardcoe that motha [censored]a was! YEAH! Oh, and Mannimarco the God.

Oh, and currently Umbra is being an extremely large pain in the ass to Clavacus Vile and even stole some of the prince's power and realm.

I need to play Battlespire.