Because I live In Hanoi, where pho comes from, eating noodles in Saigon is always an interesting experience. In Saigon nowadays, there are so many modern noodle shops which are nothing special but way out expensive, such as Pho 5 Sao , Pho 24 and Pho 2000. It's better to go to normal noodle soup stalls on the street or in the market where the atmosphere is from the past and the noodles are cheap. Real Vietnamese food is always found on the street even though it is sometimes not very comfortable to sit down on a low chair. But you can clearly see how the sellers prepare your bowl of noodles.
My breakfast of noodles was 5000Vnd noodle on the street near my hotel in Bui Vien street. It was super and the owner is a friendly lady who charges the same price for everyone, regardless of where the come from, foreigner or local.
If you want to try some other noodle soup, then go to Ben Thanh market where all types of great southern food is sold. It may a little bit expensive than on the street but there's no doubt about the quality. I always go to the Ben Thanh market whenever I am in Saigon.