All organelles are important. However, to answer your question...the most important organelle in my opinion is the nucleus. The nucleus is like the floor general or the point guard on a basketball team. It contains most of the cell's DNA, directs and controls cell function through gene expression, and mediates DNA replication during cell cycles.
Note: Not all cells have nuclei. An example of this are the red blood cells, that lack nuclei as well as mitochondria.
