Habitability is a very variable equation
so first to answer your question, probably a volcanically active planet like IO whose core is constantly squashed by the other moons and Jupiter. That would keep the crust from solidifying too much.
After that we need to create conditions where water is liquid and the atmosphere is breathable (for human habitability).
Then the question becomes what to do with that heat and volcanic atmosphere.
Like IO, put it far out in the solar system so the surface can cool faster. So the equation becomes a balance of supplementing solar heat for volcanic such that the average temperature supports liquid water.
And the atmosphere
Being a volcanic world the atmosphere wouldn't be good for photosynthetic organisms. However, plenty of terrestrial organisms are capable of making energy from volcanic environments. All humans need to survive is Air, food, and water. This is the crazy part. What if we would genetically engineer an algae capable of producing oxygen from volcanic energy. What if we could then reengineer our crops to produce edible foods from volcanic energy.
The final variable for human habitability is having a crust stable enough to produce long-term structures. This is largely an engineering problem however they would have to identify plates and build settlements towards the center as the edges would be too chaotic. These edge areas would probably be ideal for agricultural areas though as they would have more farmable energy.
This wouldn't be a lovely place for humans(as we know them) with the ash requiring masks but we could irk out an existence. With some self genetic engineering we could be adapted to thrive though.
This answer does rely a bit on the magic of genetic engineering but realistically, that's the answer to any real terraforming attempt.
Note: the engineered algae isn't too farfetched, algae was among the first organisms to survive the harsh early volcanic Earth so much so that it eventually changed the atmosphere. The only challenge here is making an algae capable of making oxygen from volcanic energy instead of sunlight. This would also be a great way to extract the harmful gases from the atmosphere as food sources for your air makers. Like our own ecosystem its likely you would need a spectrum of organisms to cleanse different toxins.