NASA's Next Goal: Mars, Titan or Comet?
NASA Plans Test of Advanced Nuclear Power Generator
My vote would go to Titan, just for the potential to see if there is other life out there in the solar system. If we were to find microbial life in the seas of titan it would be a game changer for mankind here on earth. It would help spell the end to this geocentric belief that humans have been specially placed by god and are here for a greater purpose than just to survive on a hostile world. All 3 are intriguing targets but Titan could potentially provide the most important information to the history of mankind: we are not alone.