Hashirama is strong because out of all the Senju clansmen, he's the one and only person who has the unique Wood Release KKG and the ability to control tailed beasts. The second person to inherit the title God of Shinobi.
Tobirama, he's strong and one of the greatest shinobi of all in terms of manipulation of water element and can do all Suiton jutsus with only a single hand seal and survive underwater without any near detrimental side effects.
Hiruzen has experience and served as the longest-reigning Hokage and was known as The Professor due to his apparent knowledge on all forms of ninjutsu. The third person to name him God of Shinobi and was revered as the strongest Hokage at the time of his death.
Minato was one of the few Hokages known for his impressive speed through his unique Flying Raijin jutsu and keen percertive and reflex abilities throughout the world that no shinobi, not even the current Raikage A, could ever match even today's strongest shinobi, who gained the moniker Konoha's Yellow Flash. He is also apparently a pseudo-jinchuriki, after sealing the Yin chakra half of Kurama since his death.
Apparently it's a tie for both Hashirama and Hiruzen due to being entitled as the God of Shinobi, but only one was considered to be unrivalled in terms of uniqueness and power, that's Hashirama itself.
