I’ve tried different variations of what type of bones to use in pho as well as how much. It’s not surprising that the types of bones you use as well as the quantity affects the richness and flavor of pho soup. As with all soups, this should not be different.
Of the many pho recipes on the internet, oxtail bones or beef marrow bones receive the most mentions. However, oxtail bones are not always easy to come by and are more expensive compared to the other bone scraps available. Whole Foods does sometimes sell bone marrows but they are usually frozen. I have discovered that frozen bones do not impart the rich flavor that fresh bones do. Using marrow bones by itself will give a good rich flavor but if you use marrow bones in addition to beef bones (of any parts) and use a good amount, it will result in the ultimate rich flavor that you need for a good bowl of pho.
Of all places, I found that the beef bones sold at BJs (a wholesale club) are exactly what I need although they are not always available. Unfortunately, I have to go elsewhere for the marrow bones. At one visit to the Asian grocery store, I saw some humongous beef knuckle bones that I thought was worth trying but found out that it only imparted a milder beef flavor. Other variations I have tried were marrow bones with a good big piece of brisket or marrow bones and oxtail bones. Both also are good options.
Finally, I can’t stress enough that no matter what you use, there has to be a good ratio of bones to water or the broth can taste watered down. I would suggest no more than an inch of water over the bones in the pot.