These particles mediate the fundamental particle interactions and therefore are the simplest forms of energy possible.
As for whether kinetic energy is the simplest... it's really more of a multiplier on top of existing force carriers rather than an energy on its own. For example: higher energy photons have higher momentum but it's the photon that's the carrier, it just exists at different levels.
It is far better to think about the simplest forms of energy. And the simplest would be the different force carrying particles:
http://en.wikipedia.org/wiki/Force_carrier
These particles mediate the fundamental particle interactions and therefore are the simplest forms of energy possible.
As for whether kinetic energy is the simplest... it's really more of a multiplier on top of existing force carriers rather than an energy on its own. For example: higher energy photons have higher momentum but it's the photon that's the carrier, it just exists at different levels.