The blog Future Planetary Exploration has a series of posts that present one view of the 5 most compelling planetary science missions from the list that the Planetary Science Decadal Survey is considering. The posts describe missions that would most fundamentally advance our understanding of the solar system. Using this measure, they do a good job of justifying the selection of the 4 missions described so far:
Thoughts on the Most Compelling Proposed Planetary Mission - This initial post in the series gives some background, and proposes the first of 3 missions for Mars Sample Return, the 2018 MAX-C rover, as the most compelling mission. In spite of some skepticism about technical difficulty and cost, the opportunity to take advantage of the favorable 2018 Mars launch window, the Mars Surface Laboratory team's capabilities that would otherwise be dispersed, and the ability to work with Europe's ExoMars with subsurface sample capabilities is too tempting to pass up.
Compelling Missions - Part 2 - The Venus Climate Flagship, a scaled-down version of an ambitious Venus Flagship mission concept, is presented as the second most compelling mission. This was posted a bit before the Decadal Survey list was released, so my interpretation is that it isn't so much a selection of the specific Venus Climate mission that's on the Survey's list, but rather that NASA would at least make some significant contribution to Venus studies, perhaps as part of a multinational Venus mission.
Compelling Missions 3 and 4: Icy Ocean Worlds - Missions to explore the icy Jovian moons and Saturn's Titan and Enceladus are next on the list. The preference is for the Europa Jupiter System Flagship mission and one of the Enceladus orbiter missions with significant Titan capability, but
this combination would cost almost $6B. Combine that with a $3-4B investment in Mars missions (which I predict will be the Decadal Survey's top priority) and a couple of Discovery missions, and that's pretty much the entire budget for missions next decade. I also think that the Flagship missions may face have a couple of programmatic challenges. First, NASA's last two choices for Flagship-scale missions, the Mars Science Laboratory and the James Webb Space Telescope, both experienced large cost overruns. ...
Several later posts look at lower-cost options to achieve some of the goals at the moons of Jupiter and Saturn.
While waiting for the last compelling mission, I decided to make my own series of "most compelling mission" posts with a different perspective. These are supposed to be Planetary Science missions, so science return is an appropriate measure to use to compare the various missions. However, I'd like to bring other factors into play, too.
I'd like to consider the Planetary Science missions in the context of our overall exploration and development of space. A mission that helps NASA's human spaceflight program (whether Vision for Space Exploration, Flexible Path to Mars, or other approach) and/or traditional and new commercial space efforts will have an edge in my evaluation. On the other hand, we are talking about Planetary Science, not Robotic Precursor missions. Therefore, I will stick to the Decadal Survey list, which is full of missions with high-priority science content. Planetary Science should not have to be warped beyond recognition into a substitute Robotic Precursor program just because Congress isn't wise enough to adequately fund Robotic Precursors.
For a fair comparison, I won't even consider the current 3 New Frontiers finalists (SAGE, a Venus lander mission, MoonRise, for sample return from the lunar South Pole-Aitken Basin, and OSIRIS-REx, for sample return from the asteroid 1999 RQ36), although I would otherwise be inclined to put them near or at the top of my list.
Decadal Survey: The Candy Store Posted - In order to play this game, you have to know what the proposed missions are. This Future Planetary Exploration post gives the links and information needed to find out about the missions the Decadal Survey is evaluating. The Decadal Survey reports are here. The post also includes a handy table that summarizes the missions, their anticipated launch, arrival, and end dates, and a cost estimate or cost range for each mission. Certain missions' cost estimates can be considered to be more reliable than others for various reasons (heritage, maturity of the particular mission proposal, etc), but I'll just take them as presented here.