Strictly for meat and super fast growing there's several rising hybrids like Label Rouge, that other one I heard of something Kings? Looks like heritage Rocks crossed with something else. . .
But me, I'm a serious dual purpose fan. My experience doesn't go too greatly far but I love imported Sussex, I love Araucanas (small but fast growers, very fleshy carcasses for their size, and great foragers and efficient eaters) and that's my two main favorites, though I'm dabbling in new hybrids, breeds, strains, and 2nd-3rd gen mutts each year, trying different things. I also always recommend French Marans, they grow very large and very fast, however, to me the carcass isn't the BEST out there, as the breast isn't as full and broad as other breeds. Right now I'm aiming at crossing Oriental type breeds and Sussex for that well-filled breast. Marans are very nice, but tend to have a thinner width of breastmeat. Better breeds, instead of easily grabbing a general shape of the keel, are more like Cornish X, in which you just can't grab the breast at all.
But, NO fan of Cornish X.