For a time, the strongest hero in the Dragon Ball universe was Piccolo (not Goku or Vegeta). Here’s how it happened, and how it could occur again.
For a short time in the Dragon Ball universe, Piccolo was the strongest of all the Z-Warriors, which means that he was above both Goku and Vegeta. Traditionally, the Namekian hero is several levels below the two Saiyans. He also tends to be behind Gohan, when it comes to power levels.
Early in Dragon Ball Z, Piccolo was the second strongest main character, behind only Goku. It was he who the Z-Warriors had to depend on when they were waiting for Goku during the Saiyan Saga. His importance shifted a bit when the anime made Vegeta a reluctant ally of the heroes. He slipped further in the Cell Saga with the introduction of Future Trunks and the substantial power upgrade that was given to Gohan. While Piccolo has found ways to keep climbing in power, he hasn’t been able to stay in pace with the main characters. He temporarily transcended Gohan in Dragon Ball Super but never did reach a point where he could compete with Goku and Vegeta.
However, there was a time – albeit a brief one – when Piccolo was above all the Z-Warriors. This happened during the Android Saga when Piccolo fused with Kami so that he could contribute to the battle against Android 17 and Android 18. Kami, who was originally a part of Piccolo, tremendously increased his power by merging with him. With Piccolo’s newfound strength, he was able to fight evenly with Android 17 and outclass Imperfect Cell (before his big power-up). It was said that the permanent fusion with Kami made him as powerful as a Super Saiyan. In the manga, a furious Vegeta admitted that Piccolo was ahead of him.
Of course, Piccolo didn’t get to keep his place at the top for very long. After going into the Hyperbolic Time Chamber, Goku, Vegeta, Gohan, and Future Trunks all made significant gains, and in the end, Piccolo was surpassed by all four Z-Warriors ahead of the Cell Games. While Piccolo has come a long way since those days, this was the first (and last) time that he was ahead of Goku and Vegeta. The only other situation where Piccolo was near to both of them in power was immediately after his fusion with Nail in the Frieza Saga.
The notion of Piccolo narrowing the gap again seems hard to fathom, despite the fact that he’s back to being one of the strongest Z-Warriors. Piccolo would likely defeat Majin Buu now, and he might even give Android 17 a fight. Though the Dragon Ball Super manga continuation confirmed that Piccolo’s power is still growing, he can’t catch up to Goku and Vegeta just by training. Perhaps the only way he could get on their level now is if he undergoes another Namekian fusion. That could happen if he were to fuse with the two Universe 6 Namekians that he and Gohan teamed up against in the Tournament of Power. Since the two are the fusions of every Namekian in Universe 6, they have enough power to rival Ultimate Gohan. If Piccolo had their combined power, his strength would be off even Dragon Ball’s charts.