Put these different types of radiation in order from most to least penetrating.

Alpha

These are fast moving helium atoms. They have high energy, typically in the MeV rangeDue to their large mass, they are stopped by just a few inches of air, or a piece of paper.  

Beta

These are fast moving electrons. They typically have energies in the range of a few hundred keV to several MeV.

Gamma

These are photons, just like light, except of much higher energy,They are typically from several keV to several MeV. X-Rays and gamma rays are really the same thing, the difference is how they were produced.