Only the Unicode-included Unified and six extensions considered for the question (87887 symbols, 12 compatibility included).
Possible contenders:
BabelStone Han: includes all of original map and partially all the extensions.
Source/Noto: includes all possible 65 535 physical TrueType-includable symbols, but also incomplete outside the original map.
MingLiU: circumvents the problem by having multiple files and covers all B.
What is the biggest one?
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.
