I was a big Stannis man right until he burnt his daughter. i lean towards John now albeit he's too cautious in terms of not pre-empting things although he reacts to events well. I am also a big fan of Jamie and would not be shocked if GGRM put him on the throne revealing that Games Of Thrones is really his redemption story.
I actually think it will be Danerys though. For the past season or two the show has gone to great lengths to mirror historical events and hers is the story of Aegon Targaryan. She will break the current wheel and we will see some new form of government (perhaps a regional council with Samwell and Gendry for their respective houses) but she will still be Queen and bring centuries of relative peace. I also imagine the show will end with her Dragons laying eggs.