"Unlike rocket-powered craft sent to Mars, Jupiter and other planets, Dawn is driven by a weak-but-steady ion engine. Powered by solar panels, Dawn’s engines zap a gas, xenon, with an electrical charge. As the charged gas shoots out a nozzle, it imparts a gentle push — equivalent to the weight of a sheet of paper sitting on your hand, said Mark Rayman, chief engineer for Dawn at NASA’s Jet Propulsion Laboratory in Pasadena, Calif.

But what Dawn lacks in power, it makes up in stamina. Its engines have been thrusting for some 1,000 days, steadily adding speed.

Sipping just milligrams of xenon a day, the super-efficient engines leave Dawn with enough fuel to push itself toward a second asteroid. If all goes well, next summer the 65-foot-wide craft will depart Vesta and head toward the largest asteroid in the solar system, Ceres, with arrival scheduled for 2015. Ceres intrigues scientists because it apparently holds a huge reservoir of water, Russell said."

Firstly, Dawn uses ion engines for primary propulsion, this enables it to rendezvous with two separate asteroids, something that has never been done before. Secondly, Dawn's primary science instrument is an incredible imaging spectrometer right out of star trek. It will create a high resolution global map of Vesta (and later Ceres) except at every pixel it won't merely record greyscale data or even a few color channels it'll record an entire high resolution spectrum from visible to IR wavelengths. It's secondary instrument is a gamma ray and neutron spectrometer, capable of probing the sub-surface compositon of the asteroid.

In terms of raw high-tech instrumentation, New Horizons is close, but Dawn's use of electric propulsion sets it apart.

Personally I'd rather not pick favorites, but Dawn is definitely the van guard of a stunning new generation of scientific spacecraft.