Nadd is indeed vastly more powerful as a human being. Other than spirits being pretty much confirmed to be weaker than they were in the flesh, he was also empowering Ommin and Amanoa as a spirit.
As far as I know, when Muur takes possession of someone's body, he gets access to most of his powers, but not all. So he's at least as powerful.
Ragnos was not only the most powerful of the most powerful, but when he returned as a spirit, it caused ripples in the Force that awoke Odan-Urr from half the galaxy away. To resurrect Ragnos' spirit they had to absorb a number of dark side focal points to accomplish such, it is also stated that if Marka Ragnos is to return he'd be unstoppable, implying Ragnos > Jedi Academy Luke, just like Exar Kun.
Originally posted by The Merchant
So Ragnos>JA Luke>DE Luke>Palpatine?
DE Luke isn't above Palpatine. And after DE, he began to hold back his power. Moreover, Luke thinks that every villain is unstoppable and comparable to Palpatine, while maintaining that Obi-Wan is still his better. I don't have much faith in Luke's opinions, lol.
The stuff AP said about Ragnos's Ghost causing ripples and making Odan-Urr view his funeral in a Force Vision is true, though. As is the power required to resurrect Ragnos's spirit, but then again, he had no problem manifesting in Tales of the Jedi or SWTOR. I'm assuming that it's just because Korriban's nexus was so weak at that point that the Disciples of Ragnos needed to siphon numerous Force nexuses with the Scepter to revive Ragnos's Ghost.