A Mexico City acquaintance swears that Casa Mexicana (133 Ludlow) is the most "authentic" Mexican restaurant in NYC. Despite some negative Chowhound reports on CM we wanted to give it a try, but it was closed at 5pm on Sunday so we tried Festival Mexicana (120 Rivington) instead based on a "take-out" rave in NY Mag and a couple thumbs-up on this board.
We should have just taken out--the food, especially the chicken soup, was very good, but the service was so rude we will never go back. As the only patrons we were interrupting the waitress's meal (she literally threw our food down and raced to her chow) and service for the rest of the meal ranged from hostile to nonexistent. Usually generous tippers, we left considerably less than usual.
Strawberry shake, chicken enchiladas with salsa verde, and quesadillas were fine. Chips homemade, salsa out of a can. Not worth a special trip IMO.