That would be the cell. The cell is the basic unit of organization for most living things. Living things can be classified as multicellular, with many cells, or unicellular, having only one cell. Cells are the basic unit of life in both plant and animal cells. Cells that all do the same thing are banded together to make tissue. Several tissues banded together to serve a common purpose make up an organ, such as the heart, the stomach, or the lungs. Several organs that all serve towards the same common purpose are called an organ system. Examples of organ systems would be the cardiovascular system, that has the heart, arteries, veins, capillaries, and blood, and the respiratory system, that has the lungs. Different organ systems that serve different purposes for the same effect, generally described as good health, are called an organism, such as a human being. So, cell is the lowest level, the most common denominator in this hierarchy of cell structure and organization.
In unicellular organisms, all of the life processes are performed by a single cell.
In multicellular organisms, there are 5 levels of organization.
cells -----> tissues-----> organs-----> organ systems ------> orgamism
Cells are the basic unit of structure in all living things. Examples are blood cells, muscle cells, and nerve cells.
Tissues are composed of cells which share a similar structure and function and work together to perform a specific activity. Examples are blood, muscles, and epithelial tissue.
Organs are made up of groups of tissues that perform a specific activity. Examples are the heart, brain, and stomach.
Organ Systems are composed of organs which work together to perform a specific function. Examples are the digestive system and the muscular system.
The human body contains 11 organ systems.