Edit: Accidentally double posted. xd /fixed
Even if you point out that Iruka could have some bias or that characters that have stated this don't know anything, Kishimoto himself says in every databook that Hiruzen in his prime was the most powerful Hokage. In the first databook alone, this is stated at least three times:
Page 117: "He is such a genius ninja that he was even called the 'Professor', and he still garners everyone's respect as the strongest Hokage in history." (「„プロフェッサー”と呼ばれたほどの天才忍者であり、歴代最強の火影として皆の尊敬を集め続ける。」, "'Purofessā' to yobareta hodo no tensai ninja de ari, rekidai saikyō no Hokage toshite mina no sonkei o atsume tsuzukeru")
Page 119: "'Strongest in history', 'genius', et cetera, the words used to describe the Third Hokage are filled with respect." (「„歴代最強”„天才”など、三代目火影を形容する言葉は賞賛に満ちている。」, "'Rekidai saikyō' 'tensai' nado, Sandaime Hokage o keiyōsuru kotoba wa shōsan ni michite iru.")
Page 265: "The Third is called the strongest, even among the past Hokage, and he is even admired as the 'God of Shinobi'." (「三代目は歴代火影の中でも最強と言われ、„忍の神”と謳われたことも。」, "Sandaime wa rekidai Hokage no naka demo saikyō to iware, 'Shinobi no Kami' to utawareta koto mo")