Simple, cheap, delicious. A pleasant walk around Hoan Kiem Lake you will find Pho 24 at the southern point with colourful green signage. Greeted with big smiles and welcomes, and a simple menu, ordering food is easy. A choice of beef or chicken Pho. Price is really reasonable at ₫49,000 (about £1.37) per person and you get a decent serving.
I would recommend you try this place while in Vietnam.