Experiments and simulations have found M&Ms to have a very good packing density. They achieve a 71% density in random arrangements, compared to 64% for spheres (and 74% for regularly stacked spheres).
You've got to love M&Ms ;-)
Links:
Sphere packing problem (Wikipedia).
Sphere packing problem (Mathworld).
Update:
Roland Piquepaille points out "A team of computer scientists at the Fraunhofer Institute for Algorithms and Scientific Computing (SCAI) has just developed Contpack, a 3D software to solve this problem and to maximize the volume utilization of containers".