Referring to both food and produce vendors. Here's my own experiences:
For prepared food, the tamales are my favorite, even in such admittedly inauthentic incarnations as smoked salmon and cream cheese. The sausage guy is also great, but they're not cheap. Roasted corn isn't bad. The Korean stand was disappointing, and the Peruvian was just OK. Anyone try the Hawaiian BBQ chicken?
For produce vendors, I like the stall with the bagged lettuce—it's super fresh and keeps a long time in the fridge. The heirloom tomato vendor is excellent too (her regular tomatoes are also good). The fruit place on the far North side (near Venice) has nice grapefruits and the vendor across from him, terrific unwaxed Fuji apples.
The fish truck always has a line. I'm a little leery about buying fish off a truck, but the guy seems pretty meticulous about his product, which is always well iced. I bought some snapper from him once; it was OK, but he rounded the weight up generously and somewhat overcharged me. Haven't been back.
I often shop there on Tuesdays and just walk over to Trader Joe's for what the market doesn't have—a nice alternative to supermarket shopping. Just wish the food vendors were open a little earlier than 2 pm so I could hit them for lunch. My guess is the local restaurants fought that tooth and nail:)