Alternatively, it may be based around the skills and experience of the founders, and I have certainly seen places where IAs or planning or sales or technology is the strongest muscle in the studio. And I’ve seen all of these models work brilliantly - and a couple where it wasn’t so good...
Is there a “best” way? Probably not. But there are certainly a plethora of “worst” ways. The key to getting it right isn’t about who is in charge - someone has to be and its always great if its you or at least someone in your gang and criminally terrible if it isn’t - but really, its more about finding a way for each disciple to contribute in a meaningful way.
The process can get broken because ownership is often seen as being exclusive, so that one person or team is ruling the roost, telling everyone else how its going to be done - often to quite extraordinary (and unnecessary) detail. This is very demotivating for the other disciplines which often leads to creative withering in those areas, where the inspired staff leave and are replaced by more docile characters.
A better approach is where each team member is encouraged to feel as if they co-own the project and to be actively encouraged push for the most excellent outcome possible. The trick is to live it rather than talk about it: lots of places espouse this view but it rarely actually happens because too few people really believe in it. The trick is to get everyone involved right at the start, brainstorming, talking, reviewing, and keep that going every step of the way. Of course there needs to be a leader, the one keeps the project on course, but the fact that they believe that a great idea can come from anywhere (and anyone) is what will allow brilliance to rise to the top and deliver great outcomes.