Whatever the highest Kanger coils are these days, and it takes the older square coils as well as the newer SSOCC ones.
YES the original leaks, don't know if they have fixed it, but there is a fix, before you put the end cap back on (with the coil attached) and before you fill it, grab an o-ring, drip tip ones work best, and drop it over the chimney post that the coil pushes onto, that will then seal the slight gap when the coil & base are screwed back on.
That works for mine although it is occasionally sticky at the bottom, but no real leaking, the other thing is remember to not put your pinky under the base as you will probably block the airflow, I'm always doing it