  Here we introduce the concept of "optimal particles" for strong interactions
with electromagnetic fields. We assume that a particle occupies a given
electrically small volume in space and study the required optimal relations
between the particle polarizabilities. In these optimal particles, the
inclusion shape and material are chosen so that the particles extract the
maximum possible power from given incident fields. It appears that for
different excitation scenarios the optimal particles are bianisotropic chiral,
omega, moving, and Tellegen particles. The optimal dimensions of the resonance
canonical chiral and omega particles are found analytically. Such optimal
particles have extreme properties in scattering (for example, zero
backscattering or invisibility). Planar arrays of optimal particles possess
extreme properties in reflection and transmission (e.g., total absorption or
magnetic-wall resonance), and volumetric composites of optimal particles
realize, for example, such extreme materials as the chiral nihility medium.
