B0.5V

Bet Sco A appears to have the best pedigree as B0.5V standard. 

Garcia89 lists 2 B0.5V MK standards: HD 8965 and 22951.

MK43, Hiltner69, Garrison77 & Garrison79 list Beta Sco = HD 144217 as its B0.5V standard.

Walborn72 only lists HD 36960 as a B0.5V, with 40 Per as B0.7V.
Lesh68 & Walborn90 lists HD 36960 as its B0.5V standard.

Lesh68 classifies 5 stars as B0.5V or variant: HD 24131, 35411, 36960,
144217, & 198781.

# No Standard
Keenan85 does not list a B0.5V standard

# Primary Standard
HD 144217 = Bet Sco A 
  B0.5V: MK43(stan),JM53,Bertiau58,Garrison67,Lesh68,Hiltner69(stan),Garrison77(stan),Abt81(Slettebak63),Houk88,Garrison96(for AB)
  B1V: deVauc57,Levato75
  B1: Cannon
  Jaschek64 lists B0.5IV,B0.5V,B0V,B1V
  Bet Sco B = HD 144218
  MK43 and Hiltner69 list Bet Sco as its B0.5V standard
  Fairly good agreement as B0.5V since MK43

# Secondary Standards
HD 36960 = BD-06_1234
  B0.5V: Slettebak63,Walborn71,Walborn72,Abt77,Jaschek78(Lesh68),Walborn90
  B0IV: Berger68
  B0Vp: Morgan55
  B1/2Ib/II: Houk99

HD 8965 = ALS 6596 = MCW 69
  B0.5V: JM53,Jaschek64(all),Jaschek78(Morgan55),Garcia89
  B0V: Bidelman43
  B1V: McCuskey74
  B5: Cannon

HD 22951 = 40 Per = omi Per = HR 1123
  B0.5V: JM53,Jaschek64(all),Jaschek78(Murphy69),Garcia89
  B0.7V: Walborn72
  B2: Cannon
  B1IV: Lesh68 
  B1.5IV: Abt83
  B0V: Berger55

# Weak Standards
HD 201795
  B0.7V: Walborn71(stan)
  B1V: Jaschek64(all),Jaschek78(Morgan53)
  B3: Cannon

HD 24760 = eps Per = 45 Per = ALS 7825
  B0.5V: JM53,Jaschek64(most),Jaschek78(Murphy69)
  B0.5III: Lesh68,Garcia89,MK78
  B0III: Berger62
  B1.5III: Abt83
  B1: Cannon
  Morgan switched from B0.5V (JM53) to B0.5III (MK78)

# Others
The Car
  B0.5Vp: Hiltner69,Kennedy83,Garrison96
  B0.2Vp: AbtMorgan72,Walborn76
  B0V: Hoffleit56,Whiteoak61,Jaschek78
  B0: Cannon
  O9.5V: deVauc57
  Jaschek64 lists O9.5V, B0V, B0Vp
  Houk75 lists Hiltner69 B0.5Vp
  Never proposed as a standard, and probably shouldn't be...