At this moment in time, Srinakarin Train Market (ตลาดรถไฟ ศรีนครินทร์) is my favourite night market in Bangkok. Its not as easy to reach as its namesake at Ratchada, but its more spread out and so easier to find a quiet corner. It has everything that you would need from a night market including good food and live music. It also has a lot of character. The nearest skytrain stations are at BTS On Nut and BTS Udom Suk. You can then take a songtaew or a 25 minute taxi ride from here. The market is behind Seacon Square shopping mall.
Map showing location of Srinakarin Train Market in Bangkok: