I suggest the safest and efficient way would in a perfect world be Questing. However this is not a perfect world, and our profiles aren't smart enough yet to handle every single variable thrown out there.
Questing = Not Efficient
Grinding will work very well, I am quite proud of the profile I have been working on / using. The only problem with grinding is its a dead giveaway "I bot and I haven't completed any quests"
Grinding = Efficient, but not safe.
Mixed is probably the most efficient out there IF and only IF the bot is in a stable condition. (Currently release is stable in my testing). WSG is not safe and looks extremely bottish. Arathi Basin is better but when you run in alone to a group of 6 Horde and start quickly turning around everywhere trying to kill things it raises eyebrows. AV is godly because you can easily sneak around and spend most of your time running and killing crap in large groups hiding your bottishness!
Mixed = Very Efficient if you use the right BG
Instancebuddy is going to be really smart once it gets up there but I don't think its ready yet for efficient.
Instancebuddy = Manageable probably equivalent of questing.
PVP is great and all but doing nothing but bgs since level 10 also raises eyebrows. Also I don't support WSG or Arathi Basin..
PVP = Ehh