to old city, public transport is using boat bus.
SUKHUMVIT: some street food at Sukhumvit 38 , lots of dining options, easy access to BTS, a lot of hotel, guest house, hostel and 3 main shopping malls (terminal 21 , emquartier, emporium)
SIAM: Best shopping mall area in Bangkok, plenty of food options (Thai chain restaurant and café), easy access to BTS, limited nightlife
SILOM: business area , lot of choices for great restaurants, Patpong Night Market (it is pretty quiet in 2019), easy access to BTS and MRT , easy to rich old city and Chinatown
PRATUNAM: Pratunam Market, plenty of cheap restaurants and accommodations, Platinum is the most famous shopping mall, BTS is a 15 minute walk away.
RIVERSIDE: Beautiful views, relaxed, more expensive, not too farm from downtown Bangkok can reach by sky train for 10 minutes if you stay close to BTS station. Nigh life is valid for shopping and pub at Asiatuqe. Very easy to reach Chinatown at night and old city . Easy to access pier for dinner cruise service.
CHINATOWN: Excellent street food at night, chaotic for shopping and local culture
THE OLD CITY: close to the Grand Palace and Wat Pho and flower market, at night it is a little quiet but several restaurants for good food and great views.
CHATUCHAK: Close to Chatuchak Weekend Market, not far from Don Mueang Airport. Stay at Aree and Sapankwai area with a lot of cool café and street food sale at local rate.
SUVARNABHUMI AIRPORT: Close to the airport, too far from downtown Bangkok.
Which area that I recommend to stay?
Personally, if my friend ask me where to stay, I will recommend them to stay near Aree and Sapankwai since it near me and I know a lot of yummy food around here.
Personally, I like riverside area. It is easy to access old city and be able to rich sky train which is means I can go anywhere very easily. And it is near Chinatown that I can spend at least one night to navigate street food.