The Tortoise and the Hare
The four basic forces all play central roles,
but gravity is most important for the large-scale issues of interest to cosmology.
This is because two of its basic properties
set it apart from the other forces: it is long-ranged and can act over
cosmological distances, and it always supplies an attractive force between any two
pieces of matter in the Universe (in the absence of dark energy, which
causes gravity to become repulsive).
In a scientific version of the Tortoise and Hare fable,
gravity is weak but it always wins over large
distances because it is relentless.
Thus, it is the most important force for understanding
the large scale structure and evolution of the Universe.