To a certain extent, this totally depends on the curriculum and course director at your university.
For example, the renal module at my school is PHENOMENAL. However, the respiratory module is terrible. When I use these adjectives I am referring to the quality of the syllabus, lectures, powerpoints, small-group activities, etc. So, even though there may be an organ system that you find particularly interesting, the quality of the module may make you dislike the course. Hopefully, all the modules and courses are well done at your school.
Now, as far as course content...
Favorite class: Cardiovascular module (incredibly high-yield for both Step 1 and 3rd year).
Least favorite class: Cell Biology/Histology. Seriously, in the 3rd year of middle school I looked at 0 slides and applied cell biology to patient care 0 times. Nada. Worthless.