well is true that the 1st gets put in a whole new lvl after this chapter, but at the same time so does his brother 2nd and 3rd. Madara is considered the strongest in the world but the 1st manage to beat him, so that puts him at least at the same lvl. We know that the 2nd got put as Kage while 1st was alive so this implies that 2nd was at least almost at the same lvl as the 1st. The monster would be the 3rd, both this "strongest in the world" ninjas alive and they make a 13-16 year old Kage? I dont think even the 4th was as strong as the 3rd in his prime. Based on those facts and also couple of other things:
1. 3rd wanted Jiraya as 4th but Jiraya-sama decided to follow Oro and travel the world as age told him.
2. 4th became Kage when 3rd was very much past his prime.
My opinion of ninja strengh based on fighters on their prime would be:
1. Sarutobi 3rd Hokage
2. 1st Hokage
3. Uchiha Madara
4. Jiraya and 2nd Hokage
5. 4th and maybe coming close Oro since those two were the candidates for 4th.
Just my opinion.