My vote would go to Orochimaru as being the strongest...not by a whole lot, rather it would be rather close between him and Jiraiya; but Oro winning out in the end
MMy reason why...it was Oro's life ambition to learn every single jutsu in the world, and even create some new ones of his own; which is one of the reasons he coveted the Uchihas. Given that he was such a great genius with more experience than most to boot, that he had accumulated and already learnt more jutsu than any or most shinobi by Part II; including both his Sannin counterparts.
Second, we do know that he & Jiraiya clashed one-on-one when Oro had defected from Konoha. Now this occured not too long after Hiruzen named Minato the 4th Hokage. Minato being just under 30 at the time, so either 28 or 29...it then stands to reason that both Oro &Jiraiya were atleast 14 yrs older than Minato given that Jiraiya was Minato's senei & the 12-16 yr age gap between pupil and master seems to be about standard in Naruto; So that would have put Jiraiya & Oro in their early 40s at the time they fought. Though sadly we didn't get to see the battle we do know J-man was beat. I made a point in establishing their ages cause in no way can we claim they weren't as powerful or strong back then. Someone in their 40s is well well past their prime. They were grown behind, powerful and experienced adults going at it. Not to say Jiraiay didn't learn a few tricks since; but just saying Oro already has a win
Then I look at their summons. Of all three Sannin boss summons, Oro's Manda was the most powerful. Even tho the Sannin themselves each may have had a handicap when all three last clashed there, their summons weren't yet still Manda not only was able to take on both Gamabunta and Katsuyu; but at seperate points in the fight was about to take them both out until one ran interference to save the other. So in the battle of the summons when not ganged up on Manda wins.
Then there was the battle against Naruto in Part II when Naruto unleashed his 4 tails mode. It was revealed by J-man that while facing this mode it was only the 2nd time he was faced with death. So while he was overwhelmed by Naruto in that form; Oro mmatched up not only equally but found the entire battle amusing; barely taking it seriously while in a weakening body His only regret was that the 2 yr time limit he has to stay in a body ran out at about the time. But as Kabuto described both Oro & Naruto as monsters.
So for me while the gap between Oro & Tsunade may be big; it's not that big between him and Jiraiya; but a small gap nonetheless