Following up on recommendations on this board, I finally tried Rythym & Booze at Prospect Ave and 10th Ave in Windsor Terrace this weekend. Great burger. Really, really great burger (and good fries to boot.) One of my favorite burgers in the Park Slope area. IMHO better than Bonnies (which I like) and Blue Apron (much heralded but the patty wasn't as good as the bun.)
Only thing that gives it a run for the money for me is Cocotte (although I haven't tried Stone Park Cafe's burger yet.)
R&B's burger is $7, I believe -- a good deal.