Following are mentioned the 5 strongest Dragonball Z characters. Except those that came after the transformation of Goku and Vegeta into Super Saiyan God.
Goku or as his Saiyan name says Kakarott is the strongest person on earth, and the strongest ever unfused Saiyan alive. He as an infant was sent to destroy Earth, but hit his head and forgetting his true Saiyan aggressive nature.Even as a child he defeated practically larger opponents than him. Frieza, Cell, Majin Buu, Vegeta, Super Android 17 are some characters defeated by him.
Fusing together the two most powerful Saiyans in the entire universe, Goku and Vegeta comes to be Gogeta. In Dragonball Z: Fusion Reborn Gogeta defeated Janemba with his Stardust Breaker move. He is the only Super Saiyan fusion who defeated an opponent before the fusion wore out.
The fused version of Goku and Vegeta, the two strongest Saiyans using the Potara earrings gives rise to Vegeto. First seen while fighting Buu, Vegeto’s strength greatly surpasses the strength levels of Super Saiyan 3. The anime and the manga also suggest that the strength of Vegito is even greater than that of a Super Saiyan.
Beerus is the God of Destruction, him being that clearly gives us an idea of how strong he really is. He defeated Vegeta in less than a minute, beat all the Z fighters, including Goku Super Saiyan 3. He can obliterate planets with just an ounce of his energy. Not only this, he can destroy not only planets but entire solar systems if put in a bad mood.
Akira Toriyama, the creator of Dragonball explicitly stated that Whis, the mentor and martial arts trainer of the God of destruction, Beerus is the most powerful character in the Dragonball. Whis knocked down Beerus cold with just a strike to his head. He also can make time go backward or forward 3 minutes.