Greb's peak comes way before the Ring Magazine. I think only his rights in 1925 to 1927 was factored in. He ranked 76th which is very impressive. As for Armstrong, i think many draws against unrated fighters hurt his rankings.
Basically three criteria: 1. Overall Resume 2. Peak Performance 3. Positive Rankings - Only Wins Count
Here is the Top 100 list Sugar Ray Robinson Joe Louis Muhammad Ali Toni Canzoneri Emile Griffith Floyd Mayweather Jr. Willie Pep Ezzard Charles Manny Pacquiao Archie Moore Julio Cesar Chavez Bernard Hopkins Jimmy McLarnin Roy Jones Jr. Jose Napoles Pernell Whitaker Henry Armstrong Ike Williams Sugar Ray Leonard Evander Holyfield Carlos Ortiz Alexis Arguello Marvin Hagler Young Corbett III Sammy Angott
26. Carlos Monzon 27. Oscar De La Hoya 28. Billy Conn 29. Harold Johnson 30. Dick Tiger 31. Larry Holmes 32. Jake LaMotta 33. Roberto Duran 34. Barney Ross 35. Kid Chocolate 36. Thomas Hearns 37. Canelo Alvarez 38. Louis Kaplan 39. Eder Jofre 40. Beau Jack 41. Michael Spinks 42. Lennox Lewis 43. Ismael Laguna 44. Floyd Patterson 45. Mickey Walker 46. Bob Foster 47. Jimmy Bivins 48. Peter Kane 49. Azumah Nelson 50. Vicente Saldivar
51. Jackie Fields 52. Wilfredo Gomez 53. Gene Fullmer 54. Rocky Marciano 55. Mike Tyson 56. Jack Berg 57. Shane Mosley 58. Wladimir Klitschko 59. Lou Ambers 60. Bobo Olson 61. Nonito Donaire 62. Felix Trinidad 63. Gennadiy Golovkin 64. Jack Sharkey 65. Kosta Tszyu 66. Pone Kingpetch 67. Benny Lynch 68. Ruben Olivares 69. Manual Ortiz 70. Maxie Rossenbloom 71. Tommy Loughran 72. Kid Gavilan 73. Pascual Perez 74. Salvador Sanchez 75. Haseem Hamed
76. Harry Greb 77. Marco Antonio Barrera 78. Antonio Cervantes 79. Joe Frazier 80. Aaron Pryor 81. Esteban De Jesus 82. Donald Curry 83. Roman Gonzalez 84. Andre Ward 85. Erik Morales 86. Naoya Inoue 87. Juan Manual Marquez 88. Antonio Tarver 89. James Toney 90. Terence Crawford 91. Mike McCallum 92. Carmen Basilio 93. Lou Salica 94. Wilfred Benitez 95. Joe Calzaghe 96. John Henry Lewis 97. Jose Torres 98. Gus Lesnevich 99. Lou Brouillard 100. Fidel La Barba