I like to do it before the brush; however, I read somewhere that the order does not affect the outcome. That is, as long as you do both—floss and brush—it doesn’t matter the order you prefer.

When flossing with tape or an interdental brush, I did it first, because that meant the toothpaste could get to every surface of my teeth.
Now that I use a water flosser I do it after brushing as that seems to tick both aims off at once.

I floss after brushing because I like to get the majority of anything out and then do the final detailing with the floss.

Before, that way I can rinse my mouth after flossing without making my toothpaste less effective. If I floss after, I do a quick once over with toothe paste and brush, again.