well, i finally got around to reading SF Weekly's Best Of issue (which came out in May!) and noticed their pick for Best Tamales - a place called Barrasa Market at 2840 Mission and 24th (no phone number!). my wife happened to be nearby this afternoon, so I asked her to pick us up some tamales to try.
she did find the place after some walking back and forth, it has no sign or name that she could see, just the address. and her description of it was if a dirty 7-11 carried tamales in their little hot dog steamer, it would look kinda like this. she was not impressed by the sight of it, but i assured her that they would at least be worth a try since she was there.
they spent about an hour at home, then we put them back in a steamer to reheat. we had 3 kinds - 2 "Salvadoran" with olives and potatos (one pork and one chicken), and one Mexican sweet corn tamale that was a lot like a really really good and light cornbread, with small pieces of corn kernels in the cornmeal. they were all excellent, none too heavy, wonderful flavors, good ingredients. i haven't had enough to say if these are indeed the best around, but they are better (imo) than those from La Palma, and more authentic than (though i can't say better than) the unique tamales by Primavera that we've gotten at the ferry plaza farmers market (which we love).
good luck tamale-hunting!