my birchen blue marans boy is probably the meatiest out of all my boys and i have alot of boys. if you feel his chest, it is full and he is quite stocky all the way around.
next is probably my blrwyondotte boy. he's probably the biggest boy in my whole flock. but the slowest growing. he was skin and boys until he was probably over a year old. my cochin boys don't have much breast meat compared to the marans or the wyondotte. but that could be because they are the most athletic out of all of the boys. you'll see them RUNNING EVERYWHERE trying to catch those girls, while the other boys don't. if you feel their chests, it's more fluff than meat. my next big boy is probably my RIR then my blue/black orpington boys. although my orpington boys are bigger than my RIR boy, the RIR has more breast meat than my orps.
BUT
none of these guys are hatchery birds. they are from breeders that show. so they're not your run of the mill RIR or marans. if you put a hatchery RIR next to my flock, you'll see that my RIR's are a whole size bigger than the hatchery birds. the conformations are totally different.
i had a delaware hen once and she was very nice. very meaty...but of course, nothing like the meat birds. i think my meatiest girls are my cross breeds. ones that have my RIR as a dad and either a white rock or orpington. i know that makes sex-links which aren't usually fat, but for some reason, my girls are bigger and fuller.