The answer is definitely derm, but also keep in mind that by asking "which specialties would be most conductive to me reading/studying on my own?" - assuming you are thinking specifically of residency - you are actually asking "which specialties have the least busy schedules?" Every resident is expected to study independently at home, but how much you can study at home depends in part on whether your average week is five days of 9a-3p verses six days of 6a-6p.
Basically, the answer is "anything that doesn't involve inpatient or emergency work." Which significantly limits your options, so I wouldn't go off of this metric.
If you're thinking long-term, from my personal observations the highest ratio of reading up on extremely rare stuff compared to actually seeing patients occurs with physicians who specialize in rare immunological diseases. Some major academic centers also have "sub-subspecialists" who only see patients referred to them by other subspecialists in their department and can get away with seeing small, but very involved, patient populations. I assume those people study a lot.