LF's belief is quite reasonable, since according to his information Sansa is the only legitimate child of Ned left alive. This makes her the heir as far as 90% of Westeros is concerned. There is the slight handicap of her being an accused regicide, but presumably LF has something in mind there.
Jon's claim is pretty weak. Robb's proclamation (assuming that it was what we think it was, a legitimisation and naming of Jon as heir, probably until such time as Robb himself had a son) only holds water with people who recognised Robb as a King.
Even amongst this limited group, some now have excellent reason to oppose his claim, having recanted Robb's authority in going over to the Lannisters or Stannis. So the only people who give a hoot about Robb's proclamation are those who still hold to the independence of the North and Riverlands, which might include the Blackfish, Maege Mormont and one or two others.
Also, the proclamation is limited to making Jon heir to Winterfell in the context of being heir to the King of the North and Riverlands title. He is
not heir to Winterfell in the context of being heir to the title of Lord Stark.
If you follow me.
The two things, crown and claim, are linked. He can't have the legitimacy without claiming the crown. So Jon is not in line for the Lordship at all. (Unless he takes Stannis' offer.
)
Bran is the 'real' heir. But I doubt that he will ever claim it. He's thought to be dead, and I suspect he'll allow that to stand. (Assuming he survives the series.) Either he or Rickon would face some significant obstacles in proving their claim, even though it is better than Sansa's: they would first have to prove they were who they claimed to be. (And then survive long enough to claim the title.)
Don't forget Ramsay.
His impending marriage to 'Arya' is designed to make him the
de facto heir.