First, let me explain that I do not think ANY Thai restaurant in Philadelphia is acceptable to my taste (way too Americanized). And with the exception of Lakeside Deli, I have never really liked any Cantonese restaurant here. That said:
A few weeks ago I tried Serendipity at Arch and 10th. It has a few Taiwanese dishes and the owner will make more on request. The ones I had were very good.
We tried Good Luck Cafe yesterday. As the previous poster noted, its bao are not very good. But everything else was superb, including the congee (as previously noted), and the Chinese menu has untold riches. The tip-off is that we were the only non-Chinese there. An uncomfortable restaurant with good service (see below for comparison), but in the service of the chow urge....
We also tried Cafe de Laos at 11th and Washington. Absolutely fabulous. It serves a considerable amount of Thai food and since the chef is actually from Thailand, I would imagine this to be wonderful also. Way beyond the level of the Vietnamese places I have enjoyed (including Pho Xi Lua) in sophistication and authenticity. A very comfortable restaurant by the way, with lousy service (see above for the inverse situation), at least when we went.
Were I to rate these places on 10, Lakeside would be 8, Pho Xi Lua would be 8, Serendipity would be 8 (strictly regarding the Taiwanese dishes), Good Luck would be 9 and Cafe de Laos would be 9, all things but cost considered.