Over in Vancouver I developed I five point rating system for pho. It worked pretty well, and if you are only interested in one category, say broth (undeniably the most important one), then you can just seek out the restaurant that serves the best in that category. Or if you are taking a fussy aunt or uncle out for pho, then you can go to the place with the best service and atmosphere.
Now, down to business:
Broth: Time is the key ingredient here. Bones simmered for hours will yield a broth that is discernibly meaty and rich. You also want to see a few globules of fat, but not too much. A nice but not overpowering bouquet, redolent of clove and anise, is nice and will serve to highlight the beefiness of the stock. Finally, a pho that brings these elements together in harmony with one another will transcend space and time.
Beef: Freshness, abundance, balance of meat and fat.
Noodle: Al dente is your mantra. I also look for lack of clumpiness, as well as powder, which indicates unrinsed noodles.
Condiments. Once again, I look for freshness. Freshly picked thai basil and clean, crunchy bean sprouts. A little variety is nice as well. Give me some sawtooth herb and Thai bird peppers for crying out loud!
Service & Atmosphere. No one comes to pho for silver service. But relative efficiency and a few pleasantries can elevate the experience. Pho joints often have decorative fish tanks, which can be flipping sweet, but why not keep it clean. I’d also prefer to not ask for little bowls for my sauces, they should be automatic, or come in a dispenser at the table. Wooden chopstocks? Oh no you didn’t.