Depends on what you have to choose from.  Wal-Mart and Sams (around $2.28 per pound) locally only carry select grade, but HEB carries Angus Select (a whopping $4.89 per pound).  Typically I just buy the select grades due to price and if you cook 'em right, they all turn out great.  If buying a select grade brisky, just try and get one that the thin end (flat) is pretty flexible.  If it is stiff and rigid unfrozen, I would pass that one up.  Try to get one with a nice uniform flat portion is always great for even cooking and even cutting slices.   A lot of briskets will have a real thin side to the flat and a thicker side.  They work, but the thinner side can dry out quicker.  I usually try and buy briskets in the 10-13 pound range, but if your looking to cook for a big group, 14 to 16 is not bad either, of course you could always get 2 ten pounders instead of one big one.  This will help on your cook times.  Hope this helps.  Let us know what you find.