Bangkok- best neighborhood for street food?
Hawker food is everywhere in Thailand. It would be hard to say what specific area might be best.
During the week, Convent Road, off Silom Road, has many food vendors catering to Silom's office staffs.
Thonglor has some good shops and carts: http://importfood.com/thonglor_guide.html
Aw Taw Kaw market is a good place to visit. There is plenty of offerings to be sampled. Aw Taw Kaw can be accessed by the Kamphaengphet MRT stop, or the Mo Chit BTS.
Here are some of my recs around Bangkok, with locations of those mentioned above: http://cheapeatsbangkok.com