1. Hiruzen in his prime because the manga says so, the guy knew so many jutsus. Even at old age he held his own against oro, hashirama and tobirama.
2. Hashirama could control any bijuu and more than one at the same time and he could also heal his own wounds without seals. He could use mokuton and had a scroll with many powerful jutsus.
3. Tobirama knew space/time jutsu, could draw water from thin air and use water jutsu. He also invented edo tensei and although orochimaru made it better and kabuto master it, i have no doubt tobirama would have too if it wasnt because of his untimely death.
4. Minato because he knew space/time jutsu, expert in sealing jutsus and invented the rasengan and no doubt would have mixed it with his nature transformation if it wasnt coz of his untimely death. He also had very fast reflexes and he was really good at analysis as seen against tobi. He is known as the fastest shinobi in the shinobi world.
5. Tsunade