I asked my dentist once - though I didn't have "a" dentist, I had a new senior dental school student each year, but here I refer to one who I trusted - and she said it honestly didn't matter, that any pattern that inspired me to floss regularly was the right way. Personally, I find it kind of rewarding to get all sorts of gunk out from in between my teeth when I floss, so I floss first, then brush.

One might argue that the toothbrush could then knock gunk back into those crevices, at which point it becomes clear that cleanliness is an iterative process. As in, how many passes with the whisk before you stop trying to sweep the last bits of dirt into the dustpan and just scatter them back around the room?