Who’s Buck’s most influential master ? Why ?

John Thornton is the man who rescues Buck from Hal's cruelty. He is by far the most admirable human character in the story, kind but full of the uncompromising strength that is necessary to survive on the trail. John is the ideal master because his nature is so dog-like. Thornton frees Buck from his abusive master Hal.

"As you love me, Buck. As you love me.” ch 6