The Nimmanhaemin area suffers from a serious lack of good, cheap restaurants (Best Friend Burmese being a notable exception), but Rot Yiam delivers. Their noodles come in a dark, hearty broth and you can order different cuts of beef, all of which are tender and tasty. They also serve Japanese style curry and a special kind of spring roll topped with either tamarind or curry sauce that goes great with, believe it or not, French’s yellow mustard (Guay Tiaow Lot actually does this better, but only slightly). Thai food site Moohin.com has a couple YouTube videos here and here along with an interview with the owner here (in Thai).
If you’re in need of a coffee afterwards, Impresso is just across the street.