As far as I know, Zenon's the strongest. The real one, for several reasons.
1.) She's actively hunts down Overlord. 99 Overlords and 1,000 Demon Lords IN A SINGLE NIGHT is nothing to sneeze at. Keep in mind, the average Overlord, heck, Demon Lord, can blow up a planet or two with no effort. The INSTANT Zenon wakes up, she absolutely destroys Laharl in the second game, forcing him to flee. Even Laharl himself admits he can't defeat Zenon and hightails it outta there, and this is the same guy who beat Tyrant Baal.
2.) Zenon, upon waking up, is Level 2000, without equipment. That's her most vulnerable phase, keep in mind, and yet she lays the smackdown on Laharl. Unlike most demons who reincarnate at Level 1, Zenon does this at 2000. She herself is afraid that she's so powerful she was becoming something worse than a demon. No, really. There's an entity WORSE THAN A DEMON?
3.) She, Baal and Zetta have one ability in common; transferring bodies once they are killed. Baal demonstrates this throughout the series, Zetta in the intro to Makai Kingdom and Zenon in Disgaea 2's nightmarish worst ending. Also, Zenon and Baal are so powerful they were sealed away. They couldn't be killed! Only sealed away! If you did kill them, they'd simply find a new host, and proceed to devour your nearest relatives. Hey, Gods get hungry too.
But here's the catch: After beating Baal in Disgaea 2, he doesn't possess the nearest host, unlike what Zenon does in the worst ending. His soul senses Zenon's energy and flees. In short; TYRANT BAAL FLED FROM ZENON!
4.) Her title; God of Overlords. A title so coveted yet so feared so many Overlords died just to gain her title. The only reason she lost her title was because she hated her immense power and wanted to run away from it and start a normal life, but keeps getting roped into Overlord business against her will. Poor Zenon... poor immensely powerful, cannibalistic God of a demon Zenon...
And there's my case for good old Zenon. (Please don't kill me for calling you old, Zenon... even though you're over millions of years old.) God forbid she, Zetta and Baal fought. If so, nothing will survive.
Come to think of it... NIS can make a good origin story for Zenon! Her backstory is really interesting! Make it happen guys!