Is the mitochondria the most important organelle in the body cell?

Mitochondria are certainly important but so are other organelles like ribosomes, lysosomes, Golgi apparatus, etc. It all depends on what the cells primary function is. For any one individual cell, the mitochondria probably gives way to the nucleus as it has the instructions on how to create any other organelles. However the cell would not be able to function if any of the organelles are missing.