The Sannin he fought were not the Sannin we know throughout the series, they didn't have most of their abilities than. Nonetheless, Hanzo was a feared man and could easily be put against some of the best characters.
Only the prime of his time IMO. Right now, currently in the manga, even in his prime, he's sort of fodder in the face of Top 10 Material like Edo Madara and SM Naruto. I doubt Jiraiya had Sage Mode when he fought Hanzo and he definitely had never used or even heard of Frog Song cos he was pretty clueless about it when Ma and Pa first suggested it, so I don't think Prime Hanzo was on Sage Mode Jiraiya/Nagato levels ever.
Back then, I don't think the Sannin were anywhere near as powerful as they were when the series first started. Consider that back then, Orochimaru hadn't even left the village to acquire all his test subjects and all his jutsu and become on the level of the Akatsuki. Prime Hanzo is probably on the level of a medium-level Akatsuki, with top-level Akatsuki being the likes of the 6 Paths of Pain/Nagato, Itachi and Obito.
I'd say Hanzo is easily one of the strongest shinobi around in his prime....
Those who argue that the Sanin weren't in their prime back then...how can they be in their prime at this old age?! I would imagine that they were around their twenties to thirties against Hanzo there....I'd call that their prime! Hanzo has also never been seen in his prime....he was only seen as a weakened shell against Pain and Mifune!