I don't have as much exp as a lot of other posters here, but so far:
The most rewarding stuff we do is probably the elective peds cases like flatfoot, coalitions, intoe, etc. Whether the treatments are conservative or surgical, if they get a good result, you feel like you've improved the kid's ability to fit in with peers, play sports, etc. Maybe it's because kids are innocent and all, but that feels good.
In adults, you might be suprised, but for those whose ego allows them to actually admit they like our chosen path of PO-diatry, the most rewarding cases are often conservative care - even RFC (nail care) patients. They are often older folks or persons with many health problems who are somewhat lonely and appreciate the podiatry office as much for the doc/staff socialization as they do for the medical exam + care they get. Those are honestly the patients you tend to get holiday cards/gifts, relative referrals, etc from. Surgically, arthrodesis procedures in severe OA or RA patients can work out well and improve day-to-day functionality. The complex elective recon like cavus, flatfoot, post traumatic angular deformities, Charcot, etc make for challenging cases with multi-plane deformity correction, etc... but those are honestly very tough to get optimal outomes with (a lot of things look great on the OR table or post-op XR, but follow them awhile).
...The least rewarding (for me) seems to be the trauma patients. Trauma pts are usually fairly young, active people like athletes or MVA victims who didn't expect the injury and are bitter and frustrated that they now have a temporary (or permanent if it's a severe injury) disability. A lot of trauma pts you see in teaching hospitals are also less than upstanding individuals who directly or indirectly tend to repeatedly put themselves in harm's way: substance abusers/addicts/dealers, gang members, mentally ill, criminals, general leeches of social services, etc. When you factor in those typical trauma patients' younger age (read: low income) and frequent unsavory lifestyles, you get an awful lot of noncomplaint, ungrateful, and litigious patients out of the ER... and, as you'd probably guess, a lot of them don't pay their hospital, surgeon, etc bills either.
You do get the occasional employed, grateful everyday Joe construction worker or high school athlete with a minor injury like a central met fx, ankle sprain, or a laceration that has a good outcome and the case ends up being rewarding to treat, but the majority of trauma cases reward the surgeon with little more than lint in the pocket, a huge pile of workers comp or disability forms, lost sleep, and gray hair... maybe even a certified mail attorney letter a couple years later. It's no small wonder that it's often said "trauma surgery's a young man's game."