In fact, Plato discusses his five regimes in a chronological order, not in a qualitative. In the original text, it's only suggested that there is a order of virtue. It's obvious that Plato advocates the aristocracy with the Philosopher King, but he doesn't really say something about the virtue of the other regimes.
Plato himself does not describe a theocratic state in his Politeia. I think it would've been hard to imagine such a thing in his time. Also, whilst there might not be a place for the theocratic regime in the chronological order (obviously, because it's a closed circle), something can be said about what Plato would say of theocratic states.
In Plato's state, it's very important that the Philosopher King has the real truth. If you want Plato to advocate for a theocratic state, you'll have to argue that priests have the real truth. When you argue that the (Christian, e.g.) God and the Platonic Idea of Good are actually the same thing, that would make a theocratic state pretty much the same as Plato's aristocracy indeed.
One could say that in a theocratic more people have the power, and in Plato's aristocracy there's only one (the Philosopher King). This is not true. In the original text, Plato talks about multiple Philosopher Kings:
I said: as long as the kingship does not belong to philosophers, [...], dear Glaukon, there will not be an end for the misery of states and humankind. (Plato, Politeia, 473d)
However, with a broader definition of religion, one can as well argue that a theocratic state is like the timocracy, in which the generation of Philosopher Kings includes persons of inferior nature. You can argue this, when you say religion is based on assumptions that aren't explain away in Plato's philosophy.
In general, it can't be said what a theocratic state would be in Plato's order, because it depends on the religion of the state. As long as the ultimate force of the religion tallies with Plato's Idea of Good, the theocratic state can be seen as an aristocracy. If it doesn't, the state is a timocracy, in which people that aren't aware of the Idea of Good think they have a right Idea of Good (their religion) which is actually wrong.