NOTE: For this discourse Movies 8, 9, 10, and the Bardock special will be considered canon as all CAN and DO fit into the DBZ timeline.
First of all we did have planet busters in the Saiyan saga.
Evidence: Frieza destroys planet Vegeta
Here Frieza in his first form easily charges a planet buster and takes out a planet significantly larger than Earth without even 1% of his power.
Here Frieza put some effort in and Vegeta, albeit barely matches him: Vegeta vs Frieza (First form)
Now this is pretty much right after the battle on Earth so Vegeta's power level couldn't have increased that much. This evidence makes This following claim from Vegeta in the Saiyan Saga true:Vegeta attemps to destroy Earth
Long stroy short Vegeta was
a planet buster back then.
Later in the Frieza saga Goku fights Frieza at 2% power and matches him before going Super Saiyan and affter going Super Saiyan defeats Frieza at 100% power, that's a 50x boost (consistant for all Super Saiyan levels) this tells us that Super Saiyans at base are at least Multi-planet to Solar System busters.
Fast-Foward to the ten day wait for the Cell Game and the first Broly move. In this movie with his power restricted Brolly nuked most of the South Galaxay leaving, presumbably only New planet Vegeta and its surrounding Solar System intact on Paragus' orders to keep up appearances as seen here:Destruction of South Galaxy
Note how the destruction is moving in all directions at once indicating one blast. With this evidence we can see that at full power Broly is easily a Galaxay busteer if not a multi-Galaxay buster; this leads people to belive that he is about SS3 level. This assumption is false, as Brolly is not even SS2 level.
In Movie 8 Broly fought 2 FpSSJs (Goku and Gohan), 2 USSJs (Trunks and Vegeta, albeit they were not using mtheir max power for fear of speed loss.) and a Super Namekian (Piccolo). While Broly dominates he is no where near an SS2 level. These same Z figters (minus Gohan) fought and were defeated by Cell Jrs. as easily as they were Broly and this is what an SS2 did to them:Gohan vs Cell Jrs.
Brolly did not one-shot the Z fighters as Gohan did the Cell Jrs. and therefore is not SS2 level. He is however more durable and powerful than Perfect Cell as seen here:Goku's Kamehameha against Brolly
Here's what he same attack did to Perfect Cell:Goku's Kamehameha against Cell
This shows us that Broly is more powerful than Perfect Cell, somewhere around Super Saiyan 3rd grade without loss of speed as evidenced by Cell's qoute at the end of this video here:Cell admits Trunks is stronger
and by Trunks' tranformation into USSJ
The end result is much like Broly, right down to pupiless eyes until he regains his senses. Broly is nothing more than a USSJ that lost control. Many people think that this argument is invalid because Gohan went SS2 in Movie 10: Brolly's Second Coming, this is also false. Every time Gohan went SS2 he had lightning ing in his arua: against Cell
,against Bojack in movie 9
, and at the World Tornament
. Here is Gohan's Transformation against Broly in Movie 10
, no lightning so that means he was SS1. This reinforces my point about Brolly's power, Gohan was a FPSSJ1 Goku tried the same thing against Cell but was unsuccessful and weaker than Cell unlike Trunks. If Brolly had been as strong as some claim he wouldn't have needed LSSJ to beat Gohan.
Now all of this has only been up to SS2, let's examine SS3 shall we? Majin Vegeta was an SS2 even beyond Gohan and Super Perfect Cell and he couldn't defeat Fat Buu with all his power. SS3 Goku manhandled Fat Buu. At this point SS3s are most likely Glaxay Cluster busters Mystic Gohan and above being at least multi-cluster busters. DBZ is stonger than you'd think.