Wars have been fought over the answer to that question. LOL
I follow this guide: http://www.virtualweberbullet.com/brisket4.html
Same! I do them fat side down and works great.The first brisket I ever did was fat side up and the bottom 1/8" of the meat was dried out like jerky.
I've done fat down ever since
I have done many briskets. Did my first USDA Prime (Costco) overnight last night. Gorgeous piece of meat. About 11lbs. trimmed. Fat down for 14 hours between 225 and 240. No wrap. Lifted the lid once to spritz with apple juice. Probed tender at 193 internal (middle of flat with iGrill2). The best I ever made. Maybe the best I ever had. The same sweet, dark smokiness that you get at Black's in Lockhart. Don't think I'll ever cook anything but Prime ever again.