Obviously Xehanort has gotten wrecked in one guise or another by the protagonists in practically every game in the series (excluding only a few) but now that I think about it, which version of him was the strongest?

 

I think personally Terranort was the most powerful, because almost immediately upon Xehanort's seizing the vessel he managed to give both Terra's Lingering Sentiment and Master Aqua a run for their money by himself- imagine if he had been able to have some time to hone his skills and get the hang of them!

 

It should also be noted that Ansem the Seeker of Darkness and Guardian combined were bested by Donald and Goofy merely falling on top of them in Dream Drop Distance

I'm gonna have to go with Master Xehanort. Nobody can withstand dem wrinkles

 

Honestly though, its probably young master xehanort because he's honed his time powers pretty well and he's still got his youth to help him kick ass and take names

Terranort or Young Xehanort .

Master Xehanort. Watch him rip off his shirt for the final battle in KH3, revealing that he's swole as f*ck before he wipes the floor with everybody using his bare hands.

Organization Norteen names:

 

Young Xehanort

Master Xehanort

Terranort

Seekernort

Xemnas

Rikunort

Riku SOD

Braignort

Isanort

 

From what I've seen, Master Xehanort may be the weakest, so I'd say Terranort.