Sai Jai Thai is great- in my opinion, one of the only more authentic Thai restaurants in the city. Definitely worth the trek to a somewhat sketchy neighborhood (centrally located in the Tenderloin).
Anyone that's been through the backpacker scene in Bangkok knows of the banana pancake sold on the streets- a flat bread (roti) cooked on the griddle, filled with sliced bananas and topped with condensed milk. Yum. Very rarely do you see this on the menu, but they do a great version of it here. Other notable items are the signature pork shoulder, whole fried fish with the wonderful dipping sauce, papaya salad, and mieng come (appetizer of shredded coconut, peanuts, lime, etc. wrapped up in a laloop leaf).
Some unusual and interesting items on the menu, and everything we tried was just delicious.