The worst part of medical school were the pre-clinical years. Basically just an extension of pre-med.
The best part of medical school was being a part of clinical services in the hospital. Bottom of the totem pole, yes. But, being a part of the team caring for people is very rewarding.
MS3: bottom of the totem, learning the basics: how to interview patients, how to organize your thoughts, how to develop a differential, how to enact a plan, what you provide to the treatment team is minimal, but you can have an impact and that feels good.
MS4: rotation dependent, but on rotations that you are going into, carried the team pager, expected to develop basic patient plans (Mr. Smith has a post-op fever, how do you want to manage it?) you still have minimal impact in the grand scheme of things, but generally more focused.
Internship: Back to the bottom of the totem, but now you are actually responsible for things. Nerve wracking, but exciting. What you do matters. An intern that "gets it" is worth their weight in gold. Continue to learn, but if you are good/adept, patients and people above you do notice.
Residency: More responsibility, more impact.
Personally, with the exception of M&M, things seem to only get better as I go forward.