Maria's Mexican Bistro (Union b'twn 4th & 5th) has got to be Park Slopes best Mexican restaurant. Forget Mezcals, Calexico and even the fairly decent Mexican Sandwich Company and go for the real deal over on Union.
It isn't cheap but it's well worth it-including the eight dollar guacamole which was among the best I've ever had, the chicken, apple and rasin Chile Reillnos (sp?) which was tender and delicious, a Sirloin steak in a cognac sauce and topped with a small vegetable enchilada that was rich and wonderful and the fresh grilled Salmon that was, according to my sister,quite amazing.
besides, the guys were super nice and gave us a number of gratis items.