If our understanding of Westerosi inheritance is correct, then Cersei has a stronger claim than Dany, because the line of succession never follows marriages, only blood relations. So, for example, the fact that Tommen's great grandfather married a Targaryen does not put anyone on the Targaryen side of the family into the Baratheon line of succession. Cersei is probably not the strongest claimant -- we'd have to follow the male lineages at every tier of the family tree and confirm that they're all dead, but if she's the first still-living descendant on one of Tommen's male ancestral blood lines, that's a valid claim, while Dany's is not.
There is also the fact that any claim Cersei could make, in theory, Jaime could also make, since they are twins. And under Westerosi law, male heirs always dominate, so his is stronger. It's unclear what his oaths as Kingsguard may or may not have done to such a claim.
Of course, the question is really just a pointless technicality, because:
- If you consider the Targaryens the legitimate royal family, Dany's claim is by far stronger than Cersei's because she's the last living child of the last King.
- Cersei has the throne through the simple fact that no one else is alive to contest it, and she has control over the military. No one else's claim is even remotely strong enough to convince people to back theirs over Cersei's.
In the end, the strongest claim to the throne will go to whichever manages to murder the other first.