I used 3/4 MDF for the sides, 3/4 ply for the base, and 1/2 for all other panels. With no frame, and 1x2 molding for (probably spruce or something, not pine) jointery, it is rock solid.
However - I have a friend who is on his 3rd cab using only 1/2 inch MDF and angle brackets to join everything and the cabinets seem solid. I don't know how they will stand the test of time, but they are a lot lighter. He did use 1/2" T-molding.