Depending upon how you define tall and flowering plant you may have multiple answers.
There are over 250,000 flowering plants (angiosperms). For example, certain trees such as the eucalyptus is a flowering plant. So if we are to consider the definition of a flowering plant as an angiosperm, then the Eucalyptus regnans would be the world's tallest flowering plant.
You also might be interested in learning about the Rafflesia Arnoldii, which is the largest flower by size of a single blossom and the largest unbranched inflorescence in the world, which also produces the largest tuber- is the Amorphophallus titanum. For more information about these flowers see our Everyday Mystery "What is the largest flower in the world?"
