The fastest Probe we could build now
For small values of practical an interstellar Orion craft could achieve ~3.3% c (~133 years to Alpha Centauri, one way with no deceleration at the end of the trip)). Which is in principle within reach of present technology if we chose to pursue it. But cost and aversion to nuclear technology on such an extravagant scale make it unlikely that we will see Orion space craft of any ilk built in the foreseeable future.