I don’t know if I would call Shadow the Hedgehog a "classic." The idea of turning Sonic the Hedgehog into a gun action game felt like a disconnect from all of the other games in the series. Sure, Sonic has always had an attitude, but Shadow the Hedgehog tried too hard to be edgy.
But, hey if you liked Shadow the Hedgehog you can get it for PlayStation 3 via a PlayStation Network download. Sega just re-released Shadow the Hedgehog for 1,200 yen as a PS2 Classic in Japan.