Rheumatology, from a scientific perspective, is probably one of the most fascinating fields in medicine. That said, it's probably one of the last fields I would nominate for this list.
Rheum patients tend to have chronic, debilitating, disfiguring, autoimmune diseases. In many cases there are few or no effective treatments (there's only so much that steroids, immunosuppressants, and biologics can do), and long-term use of the available medications almost inevitably leads to serious side effects. It also attracts a whole slew of fibromyalgia patients and the like, who as a group, tend to be pretty unpleasant and frustrating. Psychological comorbidities are (understandably) pretty prevalent. Yes, the little old ladies whose osteoarthritis clears up after a joint injection tend to be very grateful and pleasant, but I'm not sure they make up for the rest of the rheumatology patient cohort.
In my experience, reproductive endocrinology has the most pleasant patient base. The women who come in are generally highly motivated and extremely compliant with any treatment regimen you recommend. If you're successful, they're perpetually grateful for your help.
