[InclinedPlane]

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.

[sehugg]

More about the ion engines:

http://dawn.jpl.nasa.gov/mission/ion_prop.asp

[bfe]

As for the xenon ion engine vanguard, don't forget Deep Space One and Hayabusa.