Besides Earth? Well, it's hard to say.
I've personally held a soft spot for Saturn, not just due to the scientific facts about it but because of various nostalgic childhood educational/science-related media. In terms of science I also think we should definitely be exploring the ice giants, as we've only flown by them once and there's much we have to learn about this class of planet.
As for moons, where do I start? Our own Moon contains a geological record of the entire history of the solar system as well as much of Earth's geological history. With its proximity to Earth, it is the perfect intermediate destination and staging ground for manned interplanetary missions. In terms of scientific interest, I'd also rank Titan, as it is the only moon in a solar system with a thick atmosphere and organic molecules on its surface. Given that it likely has a subsurface ocean, there's a good chance it might possess indigenous life. The same could be said for Enceladus, though I don't know if NASA has detected any organic molecules from its plumes. Finally, the Galilean moons offer many intriguing targets for science, though Jupiter's dangerous radiation belts make exploration of that system rather difficult.
