The title of best fresh market in Bangkok is is a big claim and a hotly fought category. So firstly, what makes a great fresh market? For me it's a combination of three things: genuine local food (not catering to tourists), a wide variety of cuisine from all over Thailand and, finally, prices that reflect local tastes. So, my chosen market, and the one I live closest to is Talad (market) Din Daeng.
The market operates in two shifts, from about 4am selling fresh fruit and vegetables and meat to locals and businesses and from about 4pm onwards selling freshly cooked meals for working people on the way home.
The market is incredibly diverse, from southern Thai food influenced by Indian and Malay spices to Amok from Cambodia and of course lashings of Issan (north eastern) Thai food and much much more.
Most dishes are pre-cooked and ready to take home, but are fresh and well prepared. I feel a great sense of adventure in this place as I see a new and different dish each time I go.
In addition to take away food stalls, small road side restaurants set up later at night and cook amazingly cheap, and delicious Issan food, including Som Tam, Laap and the more unusual such as snails and frog.
The market can be reached by asking a taxi driver to take you to Talad Din Deang, or by the number 12 (orange bus) from Victory Monument. The bus is a fixed price (6.5 baht) and runs right past the market.

Kao Soi, Northern Thai food