I typed out a little rant while trying to decide.
This is pretty damn complicated. The problem is layered. It gives way to questions that need to be answered before you can even approach an objective conclusion - and if they aren't addressed, then any answer one gives is a shot of blind faith, a decision guided only by an emotional reaction to the information presented, and to the way it was presented.
Fortunately, the original question is worded pretty specifically: Put Robin, Marion, Little John, and the Sheriff in the order in which you consider they showed the most morality and honesty.
I'm pretty comfortable putting Marion at the top. Let's consider the following, remembering that honesty is indicated as weighted heavily as any other notion of morality. She starts out by pleading for their lives in the name of her love for Robin, when she's confronted by the moral dilemma. Would you allow such a personal violation and betray someone's trust in order to save their life? And not just theirs, but another person's life, as well?
The terms are not hers, they are the options presented to her. The only way she can save these two lives is by sacrificing herself and doing something despicable. But dilemma though this is, it is not our dilemma to solve, but Marion's. And her conclusion is, yes, she will do this despicable thing in order to free them.
And remember that she is "bewildered" by Robin's reaction to this. She wasn't expecting his reaction, she honestly believed she was doing the right thing. The measure of morality is doing what we think is right. And to Marion, saving two lives at the cost of herself and her love could only have been an agonizing decision.
Robin Hood comes off an ungrateful dickhead, and while he certainly being honest about his feelings, as far as actions go, well, while it's not easy for people to control their thoughts or feelings, we do have control of our actions, and you could hardly say that Robin is acting in a moral fashion here. His feelings are understandable, his outrage is forgivable, but abusing the woman who just saved his life is not justifiable.
We are, of course, given little glimpse into the background and the source of his feelings. Maybe Robin had an elaborate escape plan underway, or maybe he'd rather die than betray Marion's trust, and maybe he expected she would die before betraying his. But we aren't given any such detail, so that's speculation outside the goal of rating the morality of the actions displayed. And the test encourages us to "forget any preconceived ideas" about the characters, so let's adhere to that.
Besides, even if Marion refused to save her lover's life in the name of fidelity, there was still Little John's life at stake too. And all Little John does in this story is defend his savior while expressing his profound gratitude, which is nobler than anything we get out of Robin. The question is not about the justice of feelings, but about the display of morality, and I think in this, Little John beats Robin cleanly.
Marion heading off with John at the end says nothing about her love for Robin or the honesty of that sentiment. Loving someone and wanting to be with them are not the same thing, and nothing will drive one away from someone else like metaphorically pissing on their sacrifices.
The Sheriff I place dead last, as much of this problem stems from his shallow character. He's true to his word, in that he releases the prisoners. But honesty is more than simply honoring a single compact. The Sheriff is charged with maintaining the law and the peace of Nottingham. The two men he imprisoned were apparently dangerous enough to warrant a maximum-security cell in the dungeon, and yet he's willing to set them both free if one of their girlfriends sleeps with him.
Which means that either these two men are not actual criminals, and were imprisoned unfairly and used as blackmail by a corrupt an unjust official, or, they are in fact lawbreakers who deserve to be in jail according to the law, and were set free arbitrarily by a corrupt and unjust official.
Either way, these actions paint a dim portrait of the man, even before we account for the fact, the truth of Marion's love for Robin irrelevant, that it's hardly freely-given consent if it's extorted, that what the Sheriff did is tantamount to rape, and at the -very- least, it's a vile and repugnant way for any creature to behave, let alone a man at the helm of great power.
So my order from best to worst was Marion, Little John, Robin Hood, and the Sheriff.