Daenerys has the most obvious and least complicated claim to the Iron Throne.
If you follow the complicated lineage of that union you’ll find that Tywin Lannister (RIP, you old bastard) could make a claim to the Iron Throne through the Baratheon line.
If we consider Robert Baratheon to be a usurper, she is — as the last remaining child of Aerys II aka “The Mad King” — the legitimate successor to the Iron Throne.
I know it’s difficult to accept, but with only 13 episodes remaining in its run, Game of Thrones is barreling toward its inevitable conclusion.