Bullmastiffs

is this the best dog food

Depends on which formula you're looking at and what you consider a healthy, nutritious kibble. I try to stay away from grains (which, IMO, are fillers and offer no nutritional benefit) and brown and white rice, oatmeal, cracked pearled barley, and rice bran are all near the beginning of their ingredient list for their All Life Stages formula with flaxseed meal and millet closely following. I'm tempted to try the grain free stuff for times when raw isn't available. I wouldn't buy the All Life Stages [grain] formula, ever.

I FEED MINE THE DIAMOND PUPPY FOR LARGE BREED

I feed my Bullmastiff Eaglepack Hollistic blend, it seems to work very well with her.