Although I certainly agree that you would find much better food at most basic places in Bangkok, I wonder which in Beijing is better than Purple Haze (gongti branch)? I have not found one since the closure of Red Basil. The offerings at Purple Haze are reliable and closer to real Thai than any other place in Beijing I have been to. I am happy to be corrected on this...

Please don't say Serve the People, which although decent, is Thai-Chinese fusion. Perhaps Phrik Thai is close, but I would be interested in hearing other suggestions.