Ma Mon Luk at Quiapo is the original branch. And now I think they're down to two branches (the other one is in QC).
Ma Mon Luk was a poor Chinese who went to the Philippines to earn money and save so he can marry the woman he loves. He made a twist to the original bami being sold in Quiapo and practically reinvented Chinese cooking to suit Filipino palates. And thus, Ma Mon Luk has become an institution.
Ma Mon Luk in Quiapo is sorely lacking in ambience. People who have not been introduced to its offerings would probably grimace at the drab surroundings. Plus, it doesn't help that it's located in Quiapo and there isn't any parking space available for its patrons. But despite this, one will still see a lot of nice cars trying to squeeze into parking spaces in adjoining busy streets. Around meal times, the place still gets packed. And where take-outs are concerned, the place does make a killing. Sometimes, they even run out of siopao as early as 5 PM.
Ma Mon Luk is actually not for the poor. A regular siopao costs P40 (special costs P55, I think) already and a regular order of chicken mami costs P85 (special is P95, beef is P100). And though they have other offerings (such as short order noodles and dimsums), these two are really their bestsellers.
And seriously, I have yet to taste a yummier siopao! I actually don't know why their mami costs so much, but it is very delicious as well (but the siopao is the incomparable one).
So if you want to eat something authentic Fil-Chi, or give your expat/foreigner friends a treat, try Ma Mon Luk. If you're scared of Quiapo, try their QC branch.