Solar reference frame v-inf (i.e., extrapolated to solar system escape, otherwise this would just be a contest of who can fly closest to the sun). Gravitational binding energy relative to any massive solar system object is subtracted from the kinetic energy before converting that back to a velocity.

Resolves Prob to the number of km/s
Current record is ~16.69km/s (voyager 1 is moving at ~17km/s but the escape velocity at its distance from the sun is ~3.26km/s so the v-inf is sqrt(17^2-3.26^2) = 16.69km/s

Fission powered ion drive might achieve 2-10x the delta V of chemical rockets in deep space, but they're less able to take advantage of gravity assists and the oberth effect due to providing a slow and steady acceleration instead of a sudden burn during a close approach to jupiter etc.