Daniel writes in a recent Feser combox:

If a tenth of the attention wasted on gays, guns and gnostic government was actually spent dealings with proper Philosophy of Religion problems like PSR formulation and the MPOE Atheism would be dead and buried decades ago.

I've disputed that claim there, but here we can leave sex to the side and discuss the modal problem of evil. Last year, Michael Almeida articulated the modal problem of evil, which he thinks is the "fundamental" problem of evil, at the Prosblogion. He writes:

The challenge in the modal problem of evil is to show that, in every possible world W, if E exists in W then God co-exists with E. One natural way to take the modal problem is as asserting that there is a possible world W and some evil E in W such that God can eliminate E without a cost of a greater good G.

The problem of evil insists that there is gratuitous evil, but there can be no gratuituous evil if God exists. One line of response is to argue that it is possible that the evil in the actual world is not gratuitous.

The idea behind the modal problem of evil is to suggest that that is not enough. God is a necessary being, so if gratuitous evil is possible, then it is compossible with God, but gratuitous evil cannot be compossible with God, and thus God does not exist.

The modal problem of evil is said to be fundamental in the sense that a solution to it is a solution to every problem of evil:

Possible gratuitous evil is no less a challenge for theism than actual gratuitous evil, since God exists in every world and has the divine attributes in every world. A solution to the modal problem of evil would show that there is no possible world in which there is an instance of evil E such that God can eliminate E without the cost of a greater good. A solution to the modal problem of evil would solve all other problems of evil.

I am not convinced that this problem is any more fundamental than other problems of evil. Suppose that we are granting for the sake of argument that God exists in all possible worlds. Evidently, then, our possibility judgments are going to be conditioned on that supposition of God's necessary existence, along with his necessary goodness. To say that gratuitous evil is possible is to say that there is a possible but non-actual world in which God permits evil. By what rights can I say that that is a possible world, if I am pretending to grant the existence of a necessary and good God?

It is true, then, that the theist is committed to the impossibility of gratuitous evil. But without begging the question against the theist, the only gratuitous evil we can try to show is possible is that which is actual.