Theres one that I really like on the corner thames and Guemes, they've got a huge variety of fruits and veggies, but i also buy my sun dried tomatoes and quinoa there! pretty good prices!
There is one on Jose Antonio Cabrera (between Dorrego y Arevalo) that is well priced and often has fresh herbs as well. There is also one on the corner of Arevalo y Honduras which tends to have decent produce, but is often overpriced.