Which rank 9 gu should fang yuan refine as it's 4 th one

Now that he became a venerable through the story he has refined atleast 3 rank 9 gu - Fast Refinement gu , Heavenly Essence Treasure Imperial Lotus , Heavenly Secret Gu . So which is the 4th rank 9 gu that is most beneficial to him in your opinion or either which gu has the highest possiblity of being refined near future