[Belegorn]

If Gandalf, Radagast or anyone else would be allowed to use all their power they could overthrow him

How do you know this?

One would think listening to you that you're talking about Morgoth

Really? Okay. I'm not sure what I said for you to make such a claim.

Luthien I think it was actually challenged him in some spell battle

He was challenged many times.

she was an elf and actually challenged and deflected his spells

You're probably talking about her kinsmen Finrod who Sauron defeated and imprisoned with Beren in his dungeons. Luthien he tried to attack but the hound of Valinor, Huan, defeated Sauron. Btw, she along with Beren went before Melkor's throne and took a Silmaril from his crown. So don't try to claim that because Sauron could be bested he was somehow not a force in ME.

At the third age half of his power was in the ring

The Ring was created in the 2nd Age. His power was in it before the 3rd Age.

I think you shouldn't take what gandalf says too seriously

And here we are... I think we should not take what you say too seriously, haha.

Even if he could challenge Sauron he wouldn't

He feared Sauron. Even as Olorin. In Valinor we are told, "that he feared Sauron" [Unfinished Tales, p. 410] when he was summoned by Manwe to be a messenger to ME. In ME he tells us, "I am Gandalf, Gandalf the White, but Black is mightier still." [TT, p. 123]

Sauron isn't Morgoth

Nobody is. He was the most powerful being in Arda.

I think in this thread you make it sound as if he is

Certainly not I. You're the one who is making false claims here. First that Sauron is weak without his Ring and that people are saying he's stronger than Melkor. Now there is a thread with that question but this is not that thread.

Isildur defeated Sauron and he was human

Isildur and the Dunedain of the 2nd Age were like the Eldar in that period. They were very powerful. None of Sauron's servants could stand up to them. This is why he went off to Numenor to destroy them from within. Sauron, "could not trust even the greatest of his servants to withstand them." [Sil. p. 334] And for all his [Sauron] power "Ar-Pharazon... grew to the mightiest tyrant that had yet been in the world since the reign of Morgoth." [Sil, p. 339] So like I said, when you reference the Dunedain like Isildur and Elendil keep in mind they were more like to the Eldar than to Men. Also Sauron's body was destroyed by Elendil AND Gil-galad, the King of the Elves. Also even though he was beat, he did kill his two foes as well.

how come he was defeated when he was parted from the riing at the last alliance battle

I'm not sure I understand what you mean here. You have been saying that without his Ring Sauron is weak. When the Last Alliance beat him Sauron had his Ring. It was cut from his finger by Isildur after Sauron's body was destroyed.

you're saying he is still as powerful without the ring and that the ring just enhances his power

Yes. He is still in rapport with the Ring, unless another like Gandalf or Galadriel had taken it and wielded it. Again, maybe you think like Gandalf we should not trust to Tolkien's words. Tolkien said, "While he wore it his power on earth was actually enhanced, But even if he did not wear it, that power existed and was in 'rapport' with himself: he was not 'diminished'. Unless some other seized it and became possessed of it. If that happened, the new possessor could (if sufficiently strong and heroic by nature) challenge Sauron, become master of all he had learned or done since the making of the One Ring, and so overthrow him and usurp his place."

why did he hid himself in Dol Guldur for x years if he was a fully fleshed maia spirit at the peak of his power

"For coming out of the wastes of the East he took up his abode in the south of the forest, and slowly he grew and took shape there again; in a dark hill he made his dwelling and wrought there his sorcery, and all folk feared the Sorcerer of Dol Guldur," [Sil, pp. 371-72] This is not new. Apparently when the body is destroyed it takes time and patience for it to be rebuilt and to grow. In the 2nd Age when he was caught in the downfall of Numenor we are told, "his spirit arose out of the deep and passed as a shadow and a black wind over the sea, and came back to Middle-earth and to Mordor that was his home. There he took up again his great Ring in Barad-dur, and dwelt there, dark and silent, until he wrought himself a new guise, an image of malice and hate made visible;" [Sil, p. 347]