I've been ordering way too much takeout over the past few months, and have managed to try virtually all of the Thai places in the Davis/Porter/surrounding areas of Cambridge and Somerville. The best, by far, has been Lemon Thai Cuisine, a newish place on Highland Ave in Somerville.
They have excellent "country-style" pad thai, very tasty curries (the mango curry is awesome), and I really like their tom yum soup, which is a bit more tart than the usual takeout versions. I also enjoy their duck dishes, although I found them to be a bit less crispy than I'd like (I haven't found any really good duck at any of the places in the area, unfortunately). The only disappointment has been their fried rice, which I thought was virtually flavorless. Strange, considering the quality of the rest of their food -- I'll have to give it another try soon.
Anyway, just wanted to let those in the area know in case you haven't tried it yet.... And while I'm here, some quick reviews of a few of the other places in the area I've tried more recently:
Benjapon: Really good thai rolls. The rest of the food is surprisingly average. The pad thai was bland, but the pad kee mao was definitely acceptable (despite a bit too much oil--common problem with that dish).
Sugar and Spice: Pretty good food, but awful service both times I've eaten there, and a kind of strangely organized menu. I wouldn't rush back.
Tamarind House: Great pineapple and mango fried rices. They also have some interesting fish dishes; I really enjoyed one in particular that was some sort of fish custard made with salmon. This is also the only Thai restaurant in the area that was willing to make the food spicy enough to cause me pain (my request) -- they get bonus points for that <g>