High-temperature　heat　pumps　(HTHPs)　provide　a　powerful　solution　to　access　carbon-free　thermal　energy　from　the　renewables.　Refrigerants　have　a　central　place　in　HTHP　design.　This　study　explored　the　thermodynamic　characteristics　of　ideal　refrigerants　that　produce　the　ultimate　performance　benchmarks　defined　by　fluid　universal　behaviors　quantitatively　and　qualitatively.　A　systematic　analytic　framework　is　constructed　based　on　the　principle　of　corresponding　states.　Through　the　parametric　characterization　of　refrigerants,　the　corresponding　ideal　refrigerants　are　identified　from　the　comprehensive　refrigerant　space　by　using　a　heuristic　algorithm.　Various　operating　conditions　and　scenarios　are　incorporated　with　two　competing　indicators,　namely,　coefficient　of　performance　(COP)　and　volumetric　heating　capacity　(VHC),　employed　separately　to　determine　the　ideal　refrigerants.　A　holistic　sensitivity　analysis　of　the　characteristic　parameters　is　performed.　Moreover,　the　macro　behaviors　and　qualitative　features　of　the　ideal　refrigerants　are　also　analyzed.　Results　show　that　in　addition　to　evaluation　criteria,　subcooling　has　a　clear　influence　on　the　ideal　refrigerant　characteristics.　Under　COP,　the　ideal　refrigerants　in　small　subcooling　exhibit　a　nearly　isentropic　fluid　characteristic;　while　those　in　large　subcooling　manifest　a　dry　fluid　characteristic.　Apart　from　the　critical　temperature,　the　contrasting　preference　in　the　ideal　gas　heat　capacity　is　responsible　for　the　inherent　trade-off　between　COP　and　VHC　in　large　subcooling.