Located almost exactly halfway between Chiang Saen and Sop Ruak ('The Golden Triangle'), Jinda's Kitchen claims to have been making northern-style dishes for 50 years. They have a small menu of only about six or eight items, one of which was, of course, khao soi. Her version was decent, a little on the bland side, I must admit, but her khanom jeen nam ngiaw (unfortunately I ate it before I remember to take a pic) was fantastic, probably among the best I've consumed. Either way, it's worth a stop, as there's little in the way of good food, particularly that of the local variety, in this corner of Chiang Rai.
Jinda's Kitchen 086 654 3116 Rte 1290 (near Km 31) 7am-8pm