Why is Enceladus the only geologically active moon among its neighbours? My understanding of why Enceladus is geologically active is that tidal forces from Saturn and - to a lesser extent - from the nearby larger moon Dione provide heat to the moon's interior, just like Jupiter does for Io. 

Should the other mid-to-large-sized moons Mimas, Tethys, Dione and Rhea not also exhibit similar activity from the same forces? Granted, Dione and Rhea are further from Saturn and thus less susceptible to tidal stresses, but what about Tethys and Mimas? What special attributes does Enceladus possess that has allowed it to be geologically active in modern times? Is it purely down to the fact that the interior of Enceladus is more ice than rock compared to the other moons, or are there more factors to consider?
This puzzle is something called the Mimas paradox. The two bodies are nearly twins, but Mimas is substantially less active than Enceladus. There are currently several proposed solutions:


Mimas cooled quickly, before it entered its resonance with Tethys (which contributes to tidal heating, just as Dione does for Enceladus). A "hot start" from rapid radioactive decay (of aluminum-26 and iron-60) shortly after Enceladus' formation could have allowed its subsurface ocean to stay liquid.
Enceladus is rockier and therefore has a higher density. This would have led to quicker cooling, and would also mean that tidal dissipation could be higher for Enceladus than for Mimas. It does seem a bit paradoxical - one would imagine that more rock would mean less water ice and therefore a smaller subsurface ocean - but this isn't a problem.


It's worth noting that Tethys is thought to be made almost entirely of ice. If the rocky-Enceladus hypothesis is true, then that could be a major factor in the lack of geological activity on Tethys. However, Tethys likely once had a subsurface ocean. Dione currently does, and experiences tidal heating; however, its semi-major axis is large enough that the effects are not as strong as on Enceladus.