Probably Mormonism. AFAIK the qran says "yeah what the Bible says is true and it validates this text so that's how you know what is written here is true" even though actually reading the Bible shows it says no such thing.
The Mormon Bible however says that the Bible has been corrupted and "corrects" it in its own text.
I would say calling the Bible a corrupted work is more heretical than lying about what it says.
edit: Also to note mormons believe that if you're a good boy and do good works on earth you yourself get your own planet and become the god of that planet. That means mormons place themselves on equal footing as god if they're good enough people. That's why mormons are generally community focused and run a tight ship. Gotta get those good boy points to get your own planet when you croak.
Islam however obviously has resulted in far more evil, including the deaths of Christians. So it's worse overall.