I've only had three rotations thus far, but I'll add my very green opinion to the list:
Best:
Pedi: I'd been exposed to adult medicine during my weekly clinical experience in the first 1.5 years, and I have to say pediatric medicine is SO much better. The information is more interesting, you'll see zebras so you'll really learn to expand your differential, I felt like I was constantly learning and being taught. Add to that the fact that kids are awesome, pedi residents are really sweet, and you get to work in a very colorful, up-beat environment and pedi kicks all other rotations' @sses.
Psych: Psych residents are quirky, fun, and easy to get along with. The material is incredibly interesting--especially where psych overlaps with neuro. Whether you love it or you hate it, you have to admit that it is NOT boring. The psych EC is especially lively. You'll see your fair share of suicide attempts, crack addicts, violent crazies high on PCP, schizophrenics, and naked manic people. There are a lot of "special" people in this world, and you'll meet 'em all on psych
Worst: SURGERY
thumbdown: holy **** I hate my life right now. Hours are long, you're never that important to patient care, surgeons are jerks but also think they're God's gift to this world, **** really slides down hill and everyone is quick to remind you that you're at the bottom of that hill. To top it all off, you have to hang around and stroke the surgeons' egos or be sentenced to death by verbal hanging. I'm so sick of pretending to feel privileged to hold a big, floppy, cancerous boobie for three hours. Scrubbing in is a nightmare--especially in longer cases. Scrub nurses are Nazis about your scrub etiquette, but the surgeons could wipe their asses on the patient and no one would say anything. No moving, no touching anything, no talking depending on the surgeon, no questions, etc. Once you get used to the blood and guts, surgery itself is just not interesting. While all the surgeons are standing around being proud of themselves, you're sitting there thinking, "Yep, you sure did cut 'em good...uh huh, that was grade A cutting
" Clearly I could go on and on about this, but I'll spare you all. In summery" surgery sucks (in case you missed it).