One of the most important factors to smoking brisket is choosing a good brisket. While the word "good" is rather vague, it usually refers to the brisket having an even flat, a trim-able fat cap, and marbling throughout the meat.
Marbling in brisket refers to the intramuscular fat or fatty striations found in the lean meat. Marbling has a significant impact on how dry the lean side can become. This is why folks who smoke brisket typically go with USDA choice grade or higher.
This is also a big reason for why you should leave the fat cap intact (1/4" to 1/2") especially on the flat-side. As you eat the flat/lean side, you will get succulence from the rendered fat in each bite.
Marbling or intramuscular fat (located between/within the muscle fiber bundles) is the visible unsaturated fat that you see within the lean meat/muscle.
At cold temperatures, the fat appears white - which is in stark contrast to the lean meat. This results in a marble pattern - hence the term marbling.
It's important to note that intramuscular fat is different from intermuscular fat - the fat that exists between different muscles. This type of fat is trimmed off as it does not enhance the eating experience.
Marbling "quality" is determined through both genetics and nutrition of the steer.
To illustrate, Waygu beef is prized for it's marbling and will develop more marbling than other breeds of steer like angus. Their bloodline/pedrigree is tracked and the steer are fed carefully designed diets in order to output a desired result - in this case high amounts of marbling later in life.
Marbling has an impact on the overall eating quality of beef. It results in juiciness, moisture, tenderness, and flavor; The adage of fat is flavor truly applies here.
Increased levels of fat mean there is less collagen and lean meat/muscle per portion. This means there is less effort required for chewing. Combining this with slicing against the grain, and low and slow smoking temperatures, and you have an effortless bite.
Different countries have their own way of analyzing marbling found in beef. However, countries like the USA, Japan, and Australia make use of Human graders to visually appraise beef.
In the United States, inspection and grading are two separate programs offered by the United States Department of Agriculture (USDA). The inspection of beef for wholesomeness is mandatory and is paid for by US tax dollars.
The grading of beef is requested by beef Producers and is paid for by the Producer/Processor.
The USDA's Agricultural Marketing Service (AMS) employs 200 beef-graders who perform visual inspections and use electronic devices to appraise beef - including the distribution of marbling.
The USDA will assign one of three grades. The degree of marbling found in the beef is the primary determinant for the "quality" grade.
Anything lower than select is typically turned into ground beef.
In my opinion, USDA choice grade is best for brisket. To see a direct comparison of USDA Choice and Prime brisket, check out this article.
The main reason being that you're still getting adequate amounts of marbling throughout the lean meat without having to pay more for a prime brisket.
That's not to say that you shouldn't buy prime. If the price per pound makes sense, then by all means, buy a prime-graded brisket.
However, this is usually not the case and instead, a choice grade brisket offers the best price per pound while still having enough marbling in the lean/flat.
The reason USDA select shouldn't be used is because it doesn't have the intramuscular fat within the lean-side to prevent it from drying out. Even if you do everything right in your 12+ hour cook, if the meat lacks intramuscular fat, it's apt to still dry out.
To further complicate things, as soon as you slice the brisket, the meat will begin oxidizing which causes the flat to dry out even more. This is important to understand if you plan to bring brisket to an event or Family gathering.
Rather, keep the brisket whole. Then bring it back up to serving temperature in an oven (roughly 160F internal) before slicing.
The exact opposite could be said for the point/fatty end of the brisket. Regardless of grade, the Point end of the brisket will have no issues getting a decent slice.