For the most tender beef roasts in the crockpot try adding a cup or so of cola. It sounds strange, but it really works! It breaks down the tissue in the meat and really tenderizes it. (Which makes me wonder what that stuff does to the inside of your stomach!)
It doesn't matter what brand you use, or really even what flavor. I've used all different kinds of cola and and also Sprite & Mt. Dew. You don't taste the soda, but to be on the safe side I always dilute it about half and half with some water and I also usually add a pkg of dry onion soup mix or Au Jus. You don't have to worry about measuring anything because you really can't mess it up. Just try to turn the roast over once if you can, so it's tenderized all the way through. It works great with pork roasts too, although I would probably stick with the citrus flavors of soda rather than cola.