-----Titan Arum (Amophophallus titanium) is the world's largest flower. Originally from the island of Sumatra in the South China Sea, it was discovered in 1878 and cuttings were shipped to the Kew Gardens in London.
-----The Titan Arum plant grows 10 cm a day to a height of about two metres. When in full bloom the flower's diameter is about 1.1 metre.
-----It flowers very rarely and it first did so in 1889. The giant plant emits a sickening smell and this attracts insects to pollinate it. It is said that when the flower first bloomed in London, people fainted because of the smell.
So, back to the answer, the biggest flower in the world is Rafflesia arnoldii a very rare flower that can be found in the rainforests of Indonesia. It has the worlds largest bloom, that can grow up to 3 feet across and may weigh up to 15 pounds, has no visible leaves, roots or stem. Rafflesia arnoldii is actually a parasite that attaches to a host plant for water and nutrients and while in bloom emits an odor similar to rotten meat. This is how it attracts insects that pollinate the plant.