Call me Mr Unadventurous, but it's Earth for me, too. Especially its oceans. The fact that we know the topology of Mars pretty well but that of the bottom of our oceans hardly at all fascinates me. It seems easier to study the moons of Saturn than the obscure, cloaked depths of our own oceans.
I'd say the outer gas giants, Uranus and Neptune. All we have close up is the old Voyager photos and who know if and when the next spacecraft will be sent.
That said, I'm also looking forward to seeing the New Horizons photos of Pluto.
Saturn was my first Astronomy "Wow" as a 7 year old, but 48 years later, Jupiter fascinates me the most. Being able to watch the mechanics of a mini solar system in action with satellite and shadow transits, occultations and eclipses and appreciate what is happening, never ceases to amaze. I make good use of the 'Jupiter Moon Events' pages in the annual Astronomy Australia guide. Setting up someone to unexpectedly watch their first satellite eclipse is fun.
