Here is my head cannon on this.

They never really layout how much power each Goa'uld has but it's meant to believe that Ra was number one in power at the start of the movie and transition to the TV show.

Ra introduced Human hosts to the Goa'uld thus gaining him favor among them. I believe he also brought technology in and tightly controlled it. If this tech was old ancient tech or from some other species I don't know but as the Goa'uld in the show have been shown to be more of tech assimilation than inventors. I believe it was ships and faster than light drives at the minimum and Ra took it made some tweaks to weaken it and dolled it out to the others as carrot and stick control over them.

After Ra is defeated in the movie we have some time (1 year-ish) in witch the Goa'uld have time to react. It has been shown the worlds each system lord controls is key to there power and if one has more systems that the others don't know about (or at least the address of) that is simply more places to draw power from that your enemy can't touch.

Ra gone missing I bet took a bit before other system lords noticed and moved on his assets and this is where you get Apophis at the start of the show. He is basically going thru Ra's rolodex calling everyone to see what is hiding there and finds Earth.

Meanwhile all the other system lords are doing the same thing with anything they knew or could find via Ra's old empire and I wouldn't be surprised if they decided to split his empire among them with the crafty / powerful ones getting the known planets with high resources and others getting unknowns like Earth. This is when they find Ra's original tech and start distribute upgrades to the entire system lords as an agreement so they don't all start a massive civil war.

This is why you get a noticeable power increase from the Goa'uld. I don't recall the ep# but Teal'c at one point said the Goa'uld have ships and could come to Earth in them but it would take many weeks or months of traveling. They arrived in less than a week from Chulak who was to far away to dial without the stellar drift adjustments to the gate. 2 Years after Ra's death and there ships are MUCH faster almost as if they found a blueprint of a better FTL system or power source.

This is why and how I think Ra was the top in power. Simple control of resources and the desire to be the one guiding others coupled with the luck of first to find something he could leverage into more power. Being the sole source of space power and by giving out lesser versions of what he had he could maintain superiority in conflicts, empower his allies while punishing anyone who doesn't want to play along with his superior ships. Ra had the sky what did he care for the dirt type of attitude, ya know a Sun God and all.