Best rotations for me:
1. Internal Medicine -- GREAT rotation! You work hard, but you also learn a ton about actual medicine that is absolutely necessary for any field. In my experience, rounds are only painful if you have a bad attending/senior resident. If you have a good team, rounds can be an incredible learning experience. As a student, you develop a strong sense of responsibility towards your patients (ie, your patients are your own, and you are very involved in making diagnoses, management decisions, etc for your patients)
2. Surgery -- I had a great time in that rotation. Even though I'm not going into surgery, the skills and knowledge base developed during that rotation are crucial for any medical specialty. For example, every primary care physician (or any physician for that matter) needs to be able to recognize a surgical problem and especially a surgical emergency. Where better to learn that than on your surgery rotation seeing lots of those patients? While the OR was pretty cool, I can't imagine being in the OR half of my life. Also, many of the personalities you deal with in surgery can be quite malignant, and the hours absolutely suck.
3. Most of the Internal Medicine subspecialty rotations -- much cooler than general medicine wards. You don't get dumped on like you do in general medicine. Lots of interesting diagnostic workups, lots of procedures (in the more hands-on subspecialties), and lots of learning involved. In my mind many of the medical subspecialties are like mixing the best aspects of both general medicine and surgery. (My bias is starting to show through).
Worst rotations:
1. Ob/Gyn!!! You would think that physicians who deliver babies for a living would be some of the happiest physicians, right? Well guess again! The Ob/Gyns that I worked with make the meanest surgeons seem like harmless poodles. Their hours are awful, and it seems like they are constantly worried about lawsuits -- which might explain why they can be so unpleasant to work with. The other major thing I didn't like is that it's not very intellectual -- most of what they do seems like it's from protocol, rather than having any true rationale (again, this is mostly from my internal medicine bias). The pros of the rotation are that you do get to do a lot of procedures, and you do get to deliver babies, which is n unbelievable experience.
2. Pediatrics -- Like Lion Trees, I disliked peds because of the parents. You have to deal with parents in the rotation more than you work with the kids -- for me that took all the fun out of it. Otherwise, it's kind of like the 2 extremes of internal medicine -- the kids are either perfectly healthy and you have to reassure the worried parents that their child is fine (the majority of children that you see in clinic are like that), or the kids are incredibly sick, which is way more tragic than anyone else being sick, because they're kids. The inpatient rotation took a huge emotional toll on me for that reason, while the outpatient rotation was just plain frustrating because of the hyperworried parents.
Family medicine (which did not make either my best or worst list) is okay, but you have to REALLY like outpatient clinic. I was surprised at how much variety family practicioners really do see. Before my rotation I had assumed that most of what they do is well-patient check ups and preventative care, but I learned that it's actually the minority of their practice. The hours are also much better than most of the other specialties.