EJ mission is most ready to go, can be done quicker, and has the most mature science and technology. We are late in following Cassini-Hugyens and are losing the outer solar system science community, and we need to get going most quickly. The TS mission should be put into serious planning to be ready in 3-5 years for a new start.
Europa has been on the books for a long time as the next Flagship mission. Cassini has generated a lot of excitement over Titan but I think it needs to wait in line. Just because NASA is so slow in starting missions, is not an excuse to change science priorities.Among New Frontiers missions, I would be surprised that Io or Ganymede could make it under the cost cap. Also, what is the point of getting asteroid samples back from an NEO when we have thousands of them already in our collections? What is the incremental science? Given the amazing results from Stardust, Comet Surface Sample Return has the greatest potential for new, breakthrough science.
Here's some data on mission costs; your opinions on which mission has the best science return is a good as mine.The Io or Ganymede observers are assumed to be Jupiter orbiters that perform multiple flybys of JupiterNew Frontiers missions have approximately twice the budget for the spacecraft and instruments as Discovery missions. In the Discovery program, the Messenger Mercury orbiter and the Dawn asteroid orbiter have both be implemented. Within the New Frontiers program, two outer planet missions (New Horizons and Juno) have fit within the budget. An Io Volcano Observer has been costed at about $470M, which would include the launcher, which is just above the Discovery budge.If the instrument compliment is kept tightly controlled, then it appears to me that the a Ganymede or Io observer is not out of the question for New Frontiers. The devil, though, may be in the details of the proposals we never get to see.
A Titan mission with balloon & lander is much better for engaging the public, and 'entertains' a much broader set of scientificand engineering disciplines.