That is a loaded gun type of question. There will be so many answers, and long drawn out explanations it will take days to read. I will keep it short and sweet.
Easiest- Frame for Murder. Very few masters going to pass up the chance to kill off a minion you bait them with.
Hardest- Line in the Sand/ Distract. Both are equally hard to pull off. If it is not a problem trying to line up on the centerline and not have you opponent know right off the bat what you are doing and still pull it off, it is just getting to the center line with enough wounds left to drop the scheme. Distract is just as bad as you need to put on the condition and leave the model alive, which in turn will just wreck your face.
Honestly, I have always been disappoint with all the schemes. I was really looking forward to the hidden bluffing style of schemes that were first talked about. With the way they are now, there is little reason to hide them and barely half use scheme markers. So if there is one that uses scheme markers with three others that don't, it is pretty easy to figure out which you took. That is my take on schemes.