Great news: Biwa is now open on Sundays. I think they've got the best ramen in town: great noodles, great broth. While most people prefer the darker, shoyu-based broth, but I actually prefer the lighter, seafoody shio broth. (The ramen is an especially good deal at their late night happy hour: after 9pm, a big bowl is only $6, I think.)
The rest of their menu is great, especially the yakimono. My current favorite is the shioyaki saba, but the chicken thighs and shiitake are also up there for me. A nice sake list, too, but because they've got a full liquor license, they also offer shochu.
215 SE 9th Ave Ste 102, Portland, OR 97214