SSJ2 Gohan took Perfect Cell out of the fight with two hits. He punched him once and Cell went backwards, he punched him again and Cell couldn't move after. He was just a quivering mess and looked like he wanted to cry. He punched Cell so hard he made him vomit a whole android out.
In the above 3 images you can see that each Cell Jr was equivalent to a USSJ and was going blow to blow against Vegeta and Trunks. SSJ2 Gohan destroyed each Cell Jr with minimal effort which pretty much shows he could have killed Vegeta or Trunks in one hit each.
The point I'm trying to make is that you can't take someone who beating up a couple of SSJs and make him the strongest character in the entire series because even SSJ2 Gohan was able to replicate similar, if not more impressive fighting feats. Bearing in mind that a SSJ3 is 4x stronger than a SSJ2, how can it even be conceivable that Broly is the most powerful character in the series.
So I hope you can understand why we Broly naysayers think Broly is massively overestimated. I would have a far easier time believing his strength if he was introduced during Buu Saga and was fighting a SSJ2 Goku, Gohan and Vegeta, then I could agree and say he's easily one of the most powerful DBZ characters in the series but the fact of the matter is his feats were no more impressive than SSJ2 Gohan's.
We have no idea how he destroyed this galaxy, or over what time period and I still maintain the feat is a massive outlier compared to everything else he has shown and, just like every other DBZ movie villain, is just a big over exaggeration of his power at the start of the film to make him look scary.