G6IV

JM53, MK73, Houk88, and Keenan89 do not list a G6IV standard.

GrayNstars lists 59 Ari (HD 20618) as a G6IV standard (no parentheses). 

# Weak Standards
HD 20618 = HR 995 = 59 Ari 
  G7IV: Keenan89
  G8IV: Jaschek78(Harlan69)
  G5: Cannon
  Should be considered "G7IV" standard