Question 1: What do you think is the most important selection criterion for space science mission proposals?
Answer: There are in fact two criteria, they are:
1) The impact and ambitions of the scientific objectives, i.e. whether a mission aims at major science challenges, and the potential breakthroughs can fundamentally change human's understanding of natural laws;
2) The involvement of excellent science teams in achieving those goals, i.e. whether a mission is support by a significant number of high-quality researchers involved in analyzing the data and using the scientific observation and experiment capacity of the platform, thus producing large amounts of good science.
Where the first one is certainly more important, but it is really difficult to such proposal with feasible technical means. Therefore the second criterion comes. However, if a mission proposal meets both. It will certainly get the priority to be selected.
More reads to the question, please find in the references below:
 Ji Wu, R. Bonnet, Maximize the impacts of space science. Nature, 2017, 551: 435-436.
 Ji Wu and A. Gimenez, On the Maximization of the Science Output of Space Missions, Vol.216, Issue 1, Article 3, February, 2020
Question 2: Why space science is important to a country and how much percentage it should take in its overall space budget?
Answer: There are three different kinds of space activities for a country in civil space sector. They are Space Science, Space Technology and Space Applications. Space Science is what we are talking about, focusing on discoveries with space missions in the areas of space astronomy, solar and space physics, solar system exploration/planetary science, space earth science, fundamental physics, microgravity experiments in orbit, space life science. Space technology is investment in launcher and spacecraft technology. Space applications are operational mission for whether forecast, ocean and land remote sensing, telecommunication and broadcasting satellites, navigation satellites, etc., missions that with useful application objectives. According to the NASA budget, space science takes around 30% of its budget. At ESA, it is similar. If only space astronomy, solar and space physics and fundamental physics are concerned, it takes around 15% of this overall budget. In the developing countries like China and India, the percentage is much small, as less as 2-5%. However, as I said in the lecture, in recent years, the investment in space science in China is improving. The goal is to reach 15% by 2030.
Learn more about space activities in Chine please read:
 Brian Harvey, China in Space - The Great Leap Forward, Springer, Chichester, UK, 2013
 Bao Weimin, et al, History of Chinese Astronautics, IAA Book Series, Vol.4 History of Space, No.1, http:/shop.iaaweb.org/