The best, IMHO, is the one just East of Tesco. They have a good variety of food and lots of shopping as well. One drawback - there aren't many places to sit.
There's another night market down Sukhimvit Rd, across from the Sony store, just before you get to Laem Tong Shopping Center. This one has plenty of tables and chairs and it usually isn't as crowded as the market by Tesco.

















