This one is a head scratcher. I have compiled some data and I have to lean slightly to Hull's 90-91 season. With Rocket 44-45 and Gretz's 83-84 close behind. Bobby Hull 65-66 was the most dominate relative to the rest of the league.
Here are some stats
Name Year % Abv.2nd Goals 2nd G 2nd G name GPG
Bobby Hull 65-66 40.80% 54 32 Frank Mahovlich 0.83
Brett Hull 90-91 40.70% 86 51 Steve Yzerman/Cam Neely/Theoren Fleury 1.1
Rocket Richard 44-45 36% 50 32 Herbert Cain 1
Wayne Gretzky 83-84 35.70% 87 56 Michel Goulet 1.175
Gordie Howe 52-53 34.70% 49 32 Ted Lindsay 0.7
Gordie Howe 51-52 34.10% 47 31 Bill Mosienko 0.67
Bobby Hull 61-62 34% 50 33 Frank Mahovlich 0.714
Rocket Richard 46-47 33.33% 45 30 Roy Conacher 0.75
Phil Esposito 70-71 32.90% 76 51 Johnny Bucyk 0.974
Bobby Hull 66-67 32.70% 52 35 Stan Mikita 0.788
Charlie Conacher 34-35 30.60%36 25 Cecil Dillon 0.766
Wayne Gretzky 81-82 30.50% 92 64 Mike Bossy 1.15
Here are some stats
Name Year % Abv.2nd Goals 2nd G 2nd G name GPG
Bobby Hull 65-66 40.80% 54 32 Frank Mahovlich 0.83
Brett Hull 90-91 40.70% 86 51 Steve Yzerman/Cam Neely/Theoren Fleury 1.1
Rocket Richard 44-45 36% 50 32 Herbert Cain 1
Wayne Gretzky 83-84 35.70% 87 56 Michel Goulet 1.175
Gordie Howe 52-53 34.70% 49 32 Ted Lindsay 0.7
Gordie Howe 51-52 34.10% 47 31 Bill Mosienko 0.67
Bobby Hull 61-62 34% 50 33 Frank Mahovlich 0.714
Rocket Richard 46-47 33.33% 45 30 Roy Conacher 0.75
Phil Esposito 70-71 32.90% 76 51 Johnny Bucyk 0.974
Bobby Hull 66-67 32.70% 52 35 Stan Mikita 0.788
Charlie Conacher 34-35 30.60%36 25 Cecil Dillon 0.766
Wayne Gretzky 81-82 30.50% 92 64 Mike Bossy 1.15