William Clarke as an agent of the Mysterons deserves some credibility I think.

I'd be tempted to argue for B'lanna's uncle... Klayvor wasn't it? Granted he's a bit of a tool and I'm not 100% sure I have his name right there but he's leading a Klingon Empire which has to count for something.

Kei Morgan scoring a slot... yeah, I can see that.

And then there's the Butcher of Musashi. Sure he started out as an agent of Largo but he totally ran with that. Does he deserve to be demoted to the B-String when he had three such strong appearances (the Sonset, his capture on Salusia and the return for the Battle of Zeta Cygni)?

I've given some thought about Undocumented Features structure if it was built as a TV Series or similar.

There'd be the Original Series with three seasons
1. Formation of the WDF and their early battles, with Largo as the main adversary.
2. Sonset and Exile, running through to the Battle of Zeta Cygni. The Butcher would totally be the face of evil here with Largo in the background.
3. Future Imperfect, covering Twilight and the other stories set around the turn of the century.

And then there's the shift to Symphony of the Sword, again with 3 arcs
1. Symphonies 1 & 2 with Corwin's apothesis and the salvation of Cephiro. Akio is clearly the Big Bad here, although there's the rising problem of the Earth Alliance.
2. The Valiant tour arcs, with the Tau Ceti conflicts, the disappearance of Kei, etc. A Day of Infamy/Cybertron Dreams or 2.0 thereof.
3. The clashes with Akio's power on Oriphos, whatever's happening on Diqiu and whatever-the-hell Saionji is up to.

So getting back to the theme of the thread: I'd see Largo with the Butcher as frontman, followed by Surtur with Akio as frontmen as taking up the four front slots. Loki's certainly significant in power but he doesn't get the presentation compared to the endurance of those four as nemeses of the heroes.