Order of Power in DMC1 Universe:
1. Dante
2. Mundus
3. Charged-up Nelo Angelo
4. Nightmare
5. Griffin
Order of Power in DMC3 Universe:
1.Mundus
Mundus was most likely in hiding; observing the whole thing in silence.
Also, he was imprisoned at the time, so the prison wasn't weak enough for him to break out of yet. He had to wait for DMC1 to be able to be "set free".
Vergil ended up in his "cage" after being defeated by Dante.
Dante never met Mundus because hell is a big place, so Mundus probably wasn't there yet. It would explain why he wasn't even aware of Dante at the time. He acts like he had never met Dante before DMC1.
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2. Arkham
That breakdancing move caught all three of the "good guys" off-guard, and this was before obtaining Sparda's power. Dante needed Vergil's help just to take him down. And not just in the cutscene, either. You can't get close to Arkham without him grabbing you. That's why Vergil had to get up close to slash him -- Arkham was too focused on Dante to care about Vergil at the time.
3. DMC3 Dante
Dante didn't have Sparda's power yet because he only has the sword and one half of the amulet. Also, like I mentioned before, he needed Vergil's help to defeat Arkham.
Also, Dante had all of Beowulf's and Royalguard's moves at the end of the game. That, combined with Spiral, Kalina Ann (used in conjunction with the side-roll maneuver to take out Summoned Swords), and Rebellion, he had almost everything at his disposal to take out Vergil.
4. DMC3 Vergil
He couldn't beat Dante (in either DMC3 nor DMC1), and he probably couldn't beat Arkham on his own. And even at fully charged-up Nelo Angelo power levels, I don't think he could take on Mundus, either. I think it's only logical that I put him in fourth place.
5. Doppelganger (that was the last boss fight before Arkham)
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EDIT:
Power Levels in DmC:
Mundus
Both brothers needed to team up just to take him down. Also, he almost ripped out Dante's heart.
Dante
Waiting for sequel for improved gameplay, moveset, and animation.
Vergil
Disappointing as a boss. He should have taken his battle with Dante more seriously.
Mundus' Spawn
Dante managed to beat him on his own, plus he had just been "born". He probably had no idea what was even going on.
Dreamrunner
Good improvement on the already badass Drekavac. Not to be messed with in droves.
Drekavac
Probably one of the few enemies I could take seriously. Especially on DMD.
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Power Levels in DmC, DMC3, & DMC1
1. DMC1 Dante
2. DmC Post-VD Vergil (recently promoted to Lord of Darkness)
3. DMC1 Mundus (recently fired from Lord of Darkness position)
4. DMC1 Charged-Up Nelo Angelo
5. DmC Mundus
Both brothers needed to team up just to defeat him; also while he was considered easy to most fans, that didn't make him any less powerful.
Also, the player (DmC Dante) was only dealing with his protective outer skyscraper shell. DmC Vergil had to deal with the caramel center on the inside. That looked to have been where the real battle took place.
6. DMC3 Dante
Even with all of his power, and all six styles at his disposal, Dante would have needed help from someone else to take out DmC Mundus. It's the same reason why Dante didn't use his doppelganger in the Arkham boss fight. It would have required him to get close to DmC Mundus, and like Arkham, it just wouldn't be possible without a second real person to help him out.
7. DmC Dante
While Dante was able to punch skyscraper-sized monsters and take out Vergil without breaking a sweat, the fact that DmC 'Post-Downfall' Vergil had recently gained a monstrous load of power put him in a place that's much higher than DmC Dante, when it comes to pure strength alone.
8. DMC3 Vergil
DMC3 Vergil is fast, strong, and aggressive. However, against DmC Dante's Quicksilver DT and 8-weapon moveset, he barely loses to him in terms of power levels.
9. DmC Vergil
Yeah. He may have taken out Mundus, but he needed Dante's help in doing so. Also, he wasn't aggressive enough as a boss. Hopefully, this will be improved in the sequel with a revamped Dante Must Die mode.