>>What is the most likely monitor to be the most docile?
I can't tell you "most docile", but of the species I have experience with, flavies, savannas, and salvators have all become quite docile. Supposedly, Dumeril's are said to have a tendancy to also become docile, although the one juvenile I cared for a short time was somewhat squirmy. I have no direct experience with ackies, but they are also said to be fairly trusting.
Also, big gets really old really fast. Larger monitors are a LOT more work than smaller ones. If you think you'd like to work with one of the larger species, start with a medium sized species like a savanna, flavie, or Dumeril's before trying to deal with a salvator. Also, be aware that salvators are illegal in a number of states.
Luke