Grade of brisket?

wagyu

The grade isn’t the hugest deal for a slow cooked brisket. May not even matter at all, plenty anecdotal evidence on the net suggests it doesn’t.

Having said that it all comes down to price, at Sam’s the nominal difference in grade is reflected in the price so I usually go with prime if they have it since it is only a few dollars more.

I would be more concerned about the cut itself based on the looks. A nice uniform cut is hard to come across so if I see one I’m getting that regardless of grade.

We did a prime for Thanksgiving, I'm sure that it mattered. It was excellent. From Costco.

Depends on what you're doing with it when it's done cooking. Select brisket is fine for chopped beef sandwiches/tacos/enchiladas/whatever.

It does matter. Select has less intramuscular fat (marbling). Long, slow cooking, will cause it to dry out easier as a result. You may be able to get away with it if you were chipping it up, but it’s definitely less forgiving. It’s cheaper for a reason. Grading meat is voluntary (all meat has to be USDA inspected, but grading is not mandatory). The higher the grade, the more marbling.
Breed of cattle also matters. Black angus has bloodlines connected to wagyu breed. That’s why you pay more for CAB (certified Angus beef). This is not to be confused with red angus, which sometimes is advertised as “angus.” It’s a sleight of hand. TMI, but the bottom line is: quality in / quality out & garbage in / garbage out