I thought it was floss -> mouthwash -> brush, but either way brush was always last because the toothpaste fluoride is what you wanted left on your teeth at the end of the process with the mouthwash serving as a base layer.
Really good question.
I personally treat it like gym: it's most important that you do it, not so much in what order or even how well.
I think the most important thing is to make sure your process leaves a fluoridated product in your mouth at the end. You want the fluoride to stay on your teeth for 30 minutes.