What is the biggest font by percentage of CJK symbols already encoded?
Hanazono is a good contender:


  U+4E00  .. U+9FA5  URO 20,902 chars (fully implemented)
  U+9FA6  .. U+9FEA  URO addition 69 chars (fully implemented)
  U+3400  .. U+4DB5  Ext-A 6,582 chars (fully implemented)
  U+20000 .. U+2A6D6 Ext.B 42,711 chars (fully implemented)
  U+2A700 .. U+2B734 Ext.C 4,149 chars (fully implemented)
  U+2B740 .. U+2B81D Ext.D 222 chars (fully implemented)
  U+2B820 .. U+2CEA1 Ext.E 5,762 chars (fully implemented)
  U+2CEB0 .. U+2EBE0 Ext.F 7,473 chars (fully implemented)
  U+F900  .. U+FAD9  CJK Compatibility Ideographs 472 chars (fully implemented)
  U+2F800 .. U+2FA1D CJK Compatibility Ideographs Supplement 542 chars (fully
                     implemented)
  IVD .. (version 2016-08-15) 28,770 chars (8,828 variants included)
  (fully implemented)


Another good option would be 天珩


  CJK基本区     [4E00-9FEA] 20971字
  CJK扩展A  [3400-4DB5] 6582字
  CJK扩展B  [20000-2A6D6] 42711字
  CJK扩展C  [2A700-2B734] 4149字
  CJK扩展D  [2B740-2B81D] 222字(原先扩展D废除一字，字库中放于2A6FF，非标准码位请勿乱用)
  CJK扩展E  [2B820-2CEA1] 5762字
  CJK扩展F  [2CEB0-2EBE0]7473字
  CJK兼容   [F900-FAFF] 512码位实际472字
  CJK兼容扩展     [2F800-2FA1F] 544码位　实际542字  




Babel Stone only claims:


  53.3%


coverage with their Babel Han font.