It's clearly Danaerys Targaryen. Jon Snow is dead. Period. He's not coming back. No major characters are coming back. Even if he wasn't dead, his role would become much too important if he was Azor Ahai reborn and the commander of the night's watch, and it would also simplify the plot too much. There must be conflict between the 2 forces that stand against the Others (namely the Night's Watch and Azor Ahai)
Narrative causality is a very bad argument for Jon since Martin has worked hard to avoid it.
Also Jon is NOT the infant Targaryen (forgot the name, probably Aegon again) since he was revealed as a new character in the last book. He may however be the son of Lyanna and Rhaegar and if that's the case, he might just be part of the 3 headed dragon - read below.
Ygritte did not betray him, and he did not kill her.
Looking back at the prophecies and at what happens to Dany there is no doubt she's Azor Ahai.
She resurrected the dragons out of stone by sacrificing Khal Drogo (blood of a king) and Mirri Maz Dur (life for life, since Drogo was freshly dead) and weeping salt tears while shrouded in smoke and fire, her very survival a sign of mystic transformation - she is the only Targeryen so far to fully survive fire.
She is the blood of the Dragon (and remember dragons seem to be the opposite force of the White Walkers),
She was born on Dragonstone
Her distance from Westeros makes for better intrigue and conflict
Her "revival" into Azor Ahai has already happened at the birth of her dragons, while the Red Star was still visible, a major point in the prophecy, while Jon Snow's death happened after the Red Star is gone.
There is also a valid argument for Azor Ahai being reborn as 3 people - There are 2 living Targaryen children known of, and possibly Jon Snow
Dani's vision in the house of the undying revealed that the dragon must have 3 heads - Dani clearly one, Aegon VI (yay google) and through some shitty writing - Jon Snow
Personally I like the memento mori theme in the books, all characters are clearly mortal, even the plot-centric ones. Bringing back more characters from the dead would suck and would completely ruin the books. Catelyn is the acceptable exception - she is a secondary character and is resurrected as a completely different character - she is no longer a wife and mother (as far as she knows all her children are lost) but an avatar for vengeance.