I floss first, as recommended by my dentist and hygienist. You floss first to remove and loosen bigger bits, then when you brush the toothpaste can get in between the teeth better to further clean.
Think of it like scrubbing and washing something - first you scrub off the big bits, then you use the soap and water to really clean it. Or like first you sweep or vacuum then you wash the floors. Or fist you kick the chunks of snow/mud off your boots, then you wipe them to get any remaining bits.