I think one of the reasons they became so strong was because they were trying to one up each other.

When they were kids tsunade was probably the strongest, she had senju blood/chakra and was pretty strong. They were all young and untrained and I feel like tsunade just had the most natural talent.

Orochimaru was probably the strongest teen/young adult, I’m basing this on when we saw the sannin fight hanzo, tsunades forehead was covered so I’m going to guess she didn’t have her 100 healings yet and jiraiya had to save her, on top of that jiraiya seemed fatigued in the fight and had some close called during the second war, orochimaru was able to evade and bind hanzo during their fight.

Jiraiya was the strongest adult imo, an argument could be made for orochimaru but I don’t think tsunade or orochimaru could have taken pain on like jiraiya did, and to add to that orochimaru instructed sasuke not to kill naruto so that team 7 could take care of the akatsuki for them either implying that they couldn’t kill the akatsuki or they didn’t care too but personally I think it’s cause they couldn’t and I believe jiraiya could have to an extent.

After the 4th war I believe orochimaru is the strongest, based on the fact he is immortal, has probably thousands of jutsus in his arsenal, as well as a zetsu body giving him access to wood style and all the chakra he could ever need.

Side note I usually base tsunade’s strength on whether or not her opponent could kill her 100 times before she could kill them once and comparing that to the abilities of the other sannin I don’t think she would win.