As a complete package for a street driven performance car, the M3 stands alone.
It has an absolutely perfect combination of power, feel, smoothness, sharpness, dynamics, luxury, refinement, utility, size and looks.
The closest competitors would probably be the Audi RS4 and RS5. Both very capable. The grip of the Audis make them much easier to drive hard with a margin of safety over the M3, but what you loose is the sharpness of the M3.
The Cayman is a phenomenal car, but you give a little on power, the subjective feel of the intoxicating high-revving V8 and utility.
There just isn't any one perfect car that can dominate others in every way, but the M3 is likely as close as you can get. I am discovering this more and more every day.
Audi S6 * Audi S3 * Porsche Cayman GTS
--Former BMW M3 owner