After my rethought yesterday, I'll revamp my initial response to this instead:
1. Blade is most important, because if you have a crappy blade, nothing can save the shave.
2. Razor is next. After the blade agrees, a razor is most important. These two things are what give the shave... A great razor and blade can hide a lot of deficiencies in the other items used. Everything else is to help the razor and blade do their job. So I find a blade or two that work and use those exclusively, and I've narrowed my razors down to 3 that I use. Gives the highest probability of consistently great shaves.
3. Soap is next. Who doesn't love a great soap
I feel terrible that I'm saying soaps are third. A great soap helps before, during, and after the shave.
4. Brush is next. Frankly, in my experience, almost any brush can get you a good shave. I enjoy some more than others, yes, obviously. But it's the thing I'm least picky about. Some are beautiful, some look like a bombing survivor. Don't care either way. (Not saying I don't drool over some brush porn... I'm just saying, it isn't going to change the outcome of the shave 95% of the time.)
5. Aftershave is last. A shave isn't complete without an aftershave. But, since there aren't really any "bad" aftershaves (they all do the same exact thing, with minimal variation), I think ones choice of aftershave on a given day is mostly trivial (as long as it doesn't have ingredients that disagree... but most are safe.). Really, brushes and aftershaves are probably tied, but I'm not rewriting this again!!
* I'll leave some leeway for the outliers: if I use any one factor that's truly terrible, I suppose it could ruin the entire shave, no matter what else is used. And if there is a truly excellent razor or soap (or blade, maybe), it can hide deficiencies in the other items used. But, I don't think an excellent brush hides deficiencies in other things... that's why brushes are low in my hierarchy.