It's really hard for me to get excited by Thai restaurants in Bangkok when the street food is so often tastier, more fun to eat and inexpensive. My palate is often more Thai than a lot of Bangkokians so the idea of enduring napkins, starched service and something more bland than my taste prefers isn't my first choice of eating environment. However about a year ago I started following Jarret Wrisley on Twitter and followed the ups and downs of establishing a business in this city for a while, as he wrote for the Atlantic before creating a bar restaurant on Thong Lor. I've now eaten there a few times and the food and atmosphere has been so exceptional that I'm going to say it's the only place I can authentically recommend if guests insist on Thai.
The menu at Soulfood Mahanakorn has a care and attention to it such that newcomers wont be stretched too far, and old hands wont be left unimpressed. That's quite an unusual harmonic to achieve with Thai food in Bangkok. I'll also add that the smoked ribs are the best local style ribs I've eaten. It's not far from the Sky Train station of Thong Lor. Here's a map.