What is the most important cable?

Only you can make that decision. Depends on your gear, ears, and room. By brand synergy, do you mean all the same brand cables? If that is the case, once again, it all depends on your ears, room, and gear. If you want to tweak the system, you may have more than one brand.

If, by brand synergy, you mean one particular brand of cables mating well with one brand of electronics, I think all audiophiles want synergy between cables and components.

I would say from the cd player to your pre or amp would be the most important to begin with........

I was told the source. But I would think the speaker my resonong is that everything runs throught the speaker all your sources. From what I read keeping the cable all the same brand doesn't matter because each cable handles a specific need but I would keep the tupes the same. For example audioquest doesn't play nice with Kimber since audioquest is a solid core and kimber is not.

Me I use all the same. Or trying to.

The next step up.