Taking into account that tien is also an human than he is the most powerful human. Tien has shown various signs in the series which make others weak against him such as, he was able to fight most of the time with nappa and was able to stop imperfect cell for some minutes with his tribeam attack. And during the end of Dragonball Z Uub was considered the strongest human, capable of matching Goku in a fight (although he is a reincarnation of Buu, making his humanity debatable.) So Krillin is not the strongest human.
In Order it goes: Ubb Krillin Then Tien