Core i series cpus are the fastest cpus right now.
C2d series cpus belong to the previous generation.
Atom cpus are used in netbooks and power efficient.
In regards to Atom CPUs, they really aren't that power efficient. Here is a comparison of the Atom to C2D desktop CPUs:http://www.tomshardware.com/reviews/intel-atom-efficiency,2069-13.html
Atom CPUs are really intended for low power applications.