A rare flower known as Paris japonica has a genome 50 times bigger than humans'. With 149 billion base pairs of DNA, it's the biggest genome in the world, and stretched out end-to-end it would be taller than Big Ben.
The flower has 19 billion more base pairs than the previous record holder, the marbled lungfish, whose 130 billion base pairs weigh in at 132.83 picograms. By genomic standards, that's still utterly massive. But having such a big genome isn't actually much of an advantage - because it takes so long to replicate such a massive genome, such organisms grow very slowly and have trouble coping with the mildest environmental pressures.