I assume you're talking about which type or species of forage is best. Not to be a smart mouth, but I figure the best is whatever you have on hand or on your farm-ranch. The important thing is to manage what you already have to maximize its nutritional quality. There are limits related to climate, location, soils, forage species, etc that can limit what you can do and like they say, you can't make a silk purse out of a sow's ear. Maybe silk purses are out of your reach, but that doesn't mean you can't sell dried sow ears to dog owners so Fido has something to chew on. While that's an analogy, the point I'm making is when you are faced with an obstacle that difficult or nearly impossible to climb over, take the easy (and smart) route and go around it. For example, if you're raising stockers on tall fescue and your forage tends to not be nutritious enough during the summer to maintain weight gains, consider planting a highly nutritious and heat/drough tolerant summer annual like millet or sorghum-sudan grass. If this isn't feasable, maybe you'd be better off running a cow-calf herd rather than stockers since the former is far more tolerant (animal production wise) of lower quality forages. And regardless of what you raise, a managed (rotational) grazing system will do a lot for providing an ample supply of good quality forage when you need it. Pasture forage is by far the least expnsive feed there is if grazing suits your particular opeartion.
Maybe you have acquired pastures that were poorly managed in the past and have lots of weed pressure. Consider running sheep, especially hair sheep along with the cows; the sheep will consume and do well on many of the weeds a cow would nearly starve to death on. And hair sheep in paticular prefer legumes and weeds ove grass, meaning the sheep will compete very little with the cattle for forage. If the pasture is heavily overgrown with woody shrubs and small trees, get some goats. They will clear a pasture of small trees, briars, rose bushes, thistles, weeds, etc and eventually eat themselves out of house and home, having convereted the pasture to all grass and legumes.
You ask a rather difficult question not knowing more details of your operation. If you're running a feedlot in SW Kansas, per pound of nutrient, locally grown alfalfa may be the least expensive. If running steers in Texas, winter grazing wheat may be the ticket. In the SE U.S. it may be crabgrass in the summer and annual ryegrass over winter. During early winter in the Midwest, grazing corn stalks may be a very inexpensive alternative and reduce the amount of winter hay needed.
I've sucessfully grazed hair sheep on young cockleburrs that came up in wheat stubble, field bindweed in old hay fields, goldenrod and rose bushes as well as alfalfa, clover and grasses of every kind. I take advantage of whatever I have access to and many people don't know that when young, weeds such as pigweed, lambsquarter, waterhemp and others have nutritional quality equal to alfalfa. I do have the advantage of having hair sheep, which have eating habits more like that of a goat. In addition, the burrs of the mature cockleburr plant in stalk fields doesn't stick to hair sheep while an ordinary wool sheep becomes a 145 pound cockleburr with stickers all over.
About the only definate answer I can give you is that livestock must have roughage of some type to keep their digestive system functioning. In many feedlots where large amounts of grain is fed, that roughage can consist of wheat straw or post harvest corn stalks, both of which are extremely low in nutritional quality. The grain provides the nutrition and the stalks provide the roughage to keep things working correctly. Remember, it's much easier to swim downstream than upstream and as such, my advise is to make the best use of what you already have available.