Considering how illigetimate the last two kings have been, and that Cersai's claim to the throne is rather tenious at best, I don't think 'rightful heir' really comes into it. It's just whoever wins the fight.
I don't think anyone's arguing Cersei's claim to the throne extends beyond 'I can blow all you mofos up.'
Isn't the fact that she's sitting there at all a pie in the face to the witch's prophecies? That annoyed me.
Wasn't the witch's prophecy that she would be replaced by a younger, more beautiful queen? Everyone (including Cersei) assumed that meant Margaery, but perhaps it is actually Dany?
WRT the legitimacy of claims to the crown. Dany has the best claim as a trueblood heir of the last Targaryean king, and Robert Baratheon's claim was 1. I won the rebellion, and 2. the Baratheon family had some sort of kingly status in Westeros before the Targaryeans popped over from Valyria with their claim of 1. We won the invasion, and 2. We have dragons, mofos.
Robert was fond of a bit on the side and fathered plenty of bastards (in the TV show it was most notably Gendry who was last seen blacksmithing for the brotherhood, in the books Ned discovered there were heaps of them all over the place), but there is no legitimate claimant left in that line. Jon can be made a Stark (or a Targaryean for that matter), if declared by a noble person of the house. Roose bolton declared Ramsay his legitimate son and allowed him to have the Bolton name, shortly before regretting it immensely. Sansa is the true heir of Winterfell, so she could declare Jon a Stark, but that would mean ceding the title to him as he is older, although what's left of the Northern lords seem past that consideration anyway. Likewise, Dany could declare him a Targaryean to effectively have the whole of Westeros against the Lannisters and maybe Euron Greyjoy. Jon also has the benefit of being a bastardo of two noble bloodlines, instead of the more traditional Lord and random whore combination. She could also claim that Rhaegar and Lianna were secretly married, thus making him legitimate, but that would mean a possible contest for the throne. I think I've just gone cross-eyed.
Ten months, FFS!