I would say the tallest birds are - Malay, Shamo, Saipan junglefowl, Langhshan, Brahma, and Jersey Giant. Those would also probably be the heaviest besides a cornish X.
The HEAVIEST breed, by the Standard of Perfection is a Jersey Giant.
The TALLEST breed, by the standard, is a Malay, however Saipan Junglefowl, Shamos, and Brazilian Gamefowl also fit in there.
Cornish X are not a "breed" and their weights vary depending on feed intake, plus they don't often reach such high weights of 16+ lbs unless carefully watched and fed with a very strict diet.
I always thought Jersey Giant...but, yeah the biggest, fattest, and most spoiled in my world are definitely Silkies, too...ha. My daughter insulted my Silkie roo the other day - told him he had a big badonkadonk!
I'm still sticking with the Langshans as being one of the tallest and most readily available breeds:
(Forgive my grungy look...I was washing birds for the show)