The correct answer is Jon Snow not because he is Rhaegar"s son, The Lannister's were defeated by Daenerys and Jon killed Daenerys. In war whoever conquers becomes new King or Queen. It does not just go to the next heir.
@Bekah21. Yes you are right, there should also have been women in the choices. But if you look purely from a primogeniture standpoint, Jon/Aegon has the better claim, because he is the son of Dany's elder brother.
Stannis is the rightful ruler. Robert was king, since Tommen, Joffrey and Myrcella are not his children and Gendry is bastard, first in a row would be Stannis and second would be Renly. Targaryens lost their right to the Throne when they lost the war.
Stannis the Mannis all the way. From the standpoint of succession, he was the rightful king. Arguably, if the laws of succession were still in place, Gendry should be king. Then again, one could claim that the Baratheon dynasty ended with Tommen, maybe Cersei's death, and that no more heirs have the right. (But out of all the people in that meeting, as the now legitimate son of the last rightful king, he has the best claim)