By this i mean NO BORROWED powers, NO GIVEN powers, NO EYE TRANSPLANT, NO cheating with EDO, NO ASSISTANCE or help of any kind, NO EXPERIMENTS, NO jinchuuriki etc, you get the idea.
I think the answer is 8th gate Might Guy or Hashirama.
Hashirama is and no being a transmigration doesn't give him power up since as we know from Ashura he had to train and work hard for his power so Hashirama is as strong as he is for his own actions. He also have the power to bend Bijuus will to his own and had Bijuus 2-9 at one point and that's not borrowed or given since he used his powers to control them. Tobirama is strong as well, he was hailed as the fastest ninja in his time which means he's faster than Hashirama and Madara, is a sensor, is a genius with creating jutsus such as FTG/Edo Tensei/Shadow clone which are all forbidden jutsus, have a large chakra reserve and great intellect.
Minato is someone who fought A and Bee a couple of times and was so strong that A thought no one could surpass him while Bee trembled when Naruto mentioned Minato in their training. He is praised by Tobirama as being faster than he is, Praised by Jiraiya and Tsunade for his smarts, and was highly feared by his enemies. Without getting any known special abilities from his clan and he had a large chakra reserve (SM requires large schakra reserve) and fought many strong opponents of his time from his own power. Also had a SM at the level Naruto's SM was in Pein arc but never really used it so his base power was really strong.
Hiruzen was said to be the strongest Hokage, surpassing both Hashirama and Tobirama in his prime and knowing all of Konoha's jutsus. He had a enormous chakra reserve that was still kage level in his old age. Kabuto even goes on to say Hiruzen was the strongest of the 5 kages in his old age when the Kages at the time was Gaara dad (recently killed), A, Oonoki, and Mei.
Gaara would take the current generation as the strongest since he's using his Sand to block Susanoo sword, enton, A's leg drop, ect... Gaara is using the sand without a Bijuu and Shukaku didn't like Gaara so he wouldn't give him that power. Gaara probably have earth+wind=sand. If you take Gaara out then I'm going to go with Naruto since Kakashi said Naruto was even to or surpassing him when he has MS and Kamui and that was when his FRS was about 50% completed. Naruto's SM is gained by training and no he doesn't borrow natural energy since borrowing implies it's going to be returned and he combined it with his chakra to make a new chakra which is Senjutsu. I would say Sasuke but you took away assistance and Sasuke used drugs/forbidden jutsu to increase his speed growth so I can't count him so Neji is close next.
I named for their respective generations and put Tobirama after Hashirama in case someone say transmigration doesn't count plus after Hashirama dies he would be the next strongest. Madara was equal to Izuna when he had MS so Tobirama would go above him in my book.