Hiruzen truly is probably one of if not the strongest Shinobi to ever live, considering he never had any sort of "god" like power (Sharingan, Bijuu, Rinnegan, Ashura Chakra, Hashirama Chakra.)
-He obviously gained alot of knowledge by being the 2nd's student, the creator of so many techniques, but is also a master at strategy and very quick learner on new opponents(as displayed vs jubito.)
-His Staff while is a nice weapon, isn't exactly needed for Hiruzen, or atleast when he was prob in his prime(imo.)
-And the ability alone to have mastered all 5 elements and can use them at the same time is almost godlike itself.
TLDR: Hiruzen is probably the strongest Shinobi to ever live without any sort of "god like" power to boost his natural talent.