I'm not a fan of that term. We can't make a judgement because we've seen very little from him in 'Prime' form. Undoubtedly he was a powerful shinobi but most of what we knew about him came long before the storyline had fully developed. Prime Hiruzen certainly causes flame wars because of Hashirama's new found tank-ery.
It's true we've seen nothing from prime Hiruzen, but we do know that he had large chakra reserves, large enough that his primary mode of fighting was with kage bunshins. That alone tells us there's many jutsu he would probably be able to use making him a formidable opponent. However, prime Hiruzen doesn't have old Hiruzen's experience, so that could be why old Hiruzen might actually be more powerful. Even in Hiruzen's old age, his chakra levels were average. That's not a weakness given that he had excellent chakra control. Combined with his experience, it makes him a dangerous opponent to go against.
Well logically I'd view it just as Prime Hiruzen>3 sannin. But other than that, idk. I would assume that he can at least be on the same level as minato or itachi, if not higher. I doubt he is stronger than Nagato, but could probably give him trouble. Definitely not stronger than Hashi, and also prob not stronger than Kabuto either.
That's just the way I view that term.
I also doubt Hiruzen can defeat Nagato, but defeating Pain is possible. Jiraiya was able to to at least defeat three paths, so I expect Hiruzen to be able to do the same. Defeating all 6, I don't know. If Jiraiya stood a chance of defeating Pain, Hiruzen would have a higher chance, but against Nagato, it's in Nagato's favor, but Hiruzen will definitely give him a challenge.
I like the character but "Prime Hizuren" is a groundless arguement. Half of his fanboys rely on Emna too. One of the sources came from him too. Ego-maniac anyone?
Prime Hiruzen has larger chakra reserves and better stamina so he'll be able to fight longer, that's all fanboys rely on. One can, however, make an argument that old Hiruzen is better than prime Hiruzen because old Hiruzen actually has experience to aid in his battles, prime Hiruzen only has chakra levels and stamina.
Prime Hiruzen is under the spectrum of high kage IMO - he can be underrated or overrated depending. Some people say that he stomps legends of Hashirama, but others say that he would get defeated by any of the current 5 kage.
He was obviously meant to be quite powerful. It's just hard to grasp because his actual feats were shown in Part 1 and we haven't seen his Prime self in action.
Hiruzen would stomp on Tsunade, 4th Raikage, 3rd Tsuchikage, and 5th Mizukage. He was the strongest of the 5 Kage at the time of his death. Being so old and able to be more powerful than the 4th Raikage and 5th Mizukage, two young opponents who have much larger chakra reserves, is incredible.
Prime Hiruzen should never be used in any arguments because we know nothing about him.
We do know that he has larger chakra reserves and stamina, but that's about it.