I've been out of the theoretical physics game a little while, what are the leading "untestable" quantum-gravity theories out there today?

Supersymmetry, supra-dimensionality, string theory, twistor theory, M-theory (which is a superset of string theory).

String theory's one.

String Theory is a theory in the same sense that Quantum Field Theory is a theory. You need concrete phenomenological models to make predictions. While there some ways to do that starting from string theory there is no one known model to rule them all, the way the earlier attempts like Georgie‘s SU(5) GUT Model was. For some flavors like F-theory it is even a research questions how to make certain predictions of matter content etc at all. This is made more uncomfortable by te fact that the most natural susy extensions of the standard model have been all but ruled out. This also eliminates one of the main reasons for susy: as a solution to the fine tuning problem.

To be fair, the parent asked for untestable theories.

String theory's several trillion.

True, I should say "a" string theory.