If Hailee Steinfeld wasn't already the queen of Bills Mafia ... someone better get her a crown now -- 'cause she just roasted Buffalo's biggest rivals!!

Josh Allen's fiancée fired off the savage burn during a game of "Truth or Wear" with Who What Wear on Wednesday ... after she was asked to take a "football IQ quiz."

Part of the test required her to list off everyone in the Bills' AFC East division ... but rather than go through the Patriots, Dolphins and Jets by name -- she went a route Buffalo fans will adore.

"You've got the Buffalo Bills," she said, "and the Bills' three sons!"

If that wasn't enough to endear her to the Buffalo faithful, her answers to other Allen-related topics certainly will.

She called the 28-year-old quarterback "the man of my dreams" ... and added that his proposal to her a few months ago in Southern California was "the most perfect proposal I could have ever, ever dreamt."

Allen and Steinfeld have been dating for about two years now ... but they've really given the public -- and Bills Mafia -- a better look into their relationship since the summer.