Oro, of course. why do u underestimate oro's abilities. do u know in the 1st part he posed the single most greatest threat to konoha. His hobby was also learning kinjutsus. He was the 2nd shinobi to learn to use impure world resurrection. he had kusanagi which would have cut thru gaara's sand like knife thru butter. Even in a weakened state he toyed with 4tailed naruto. He was most probably the strongest of the sanin.
Fans tend to disregard him because of the scene showing him being easily defeated by itachi which i think is an unfair portrayal. Personally, i don't think itachi will even find it easy against any of the 3 sannin and i'm sure jiraiya would have defeated itachi in a battle. Oro's only weakness was his phobia of death and his obsession with conquering it.