I think Science Diet (and many other foods) are way over marketed...if that makes sense. They may not be the best food(s) but clearly have a huge, effective marketing budget!
And yes, sadly many vets have very limited in depth knowledge regarding proper nutrition. (But some DO.)
That being said, it is not the 'best food' but my be the 'best food' for some dogs. Not all dogs can eat the high end brands. Whatever food is chosen to feed a dog, it's important that the diet be supplemented to add anything that the food lacks.
Every brand dog food from the low end to the high rated are going to market their product to SELL (because that's what companies do), claiming it to be the best made of only the best ingredients. Don't be fooled by fancy claims and websites oozing catch phrases like "holistic" and "organic" None of them are going to say, "we make dog food out of crap" which is the case for MOST brands.