On a personal level, probably the bit between starting the degree and ending it...
Nah but really. First year was definitely hardest for me. I struggled a lot with moving away from home, hugely increased workload, making friends etc. Became extremely depressed, went on antidepressants, failed an exam and an essay... it was all very difficult, but after that things got better.
Clinically there's only one case that ever affected me and it was an odd one. I was spending the day with on-call ICU as a 5th year and got called down to resus for a lady in pulmonary oedema. She was on the older side, somewhat delirious and agitated, and really really unwell. I don't really remember the details any more, just different doctors and nurses coming and going. I just stuck with the patient, helping the different A&E nurses hold her down to take bloods etc, and spent ages just chatting to her son and reassuring him as best I could with my limited knowledge. The ICU consultant came down and had the 'let's focus on comfort' talk. The son was very understanding and reasonable. I spent hours down there, well into the evening. And when I left her son thanked me profusely for everything I had done. And for some reason that deeply upset me. I felt so useless, and unworthy. I had literally been holding his mother down, and just listening. I had done NOTHING. And I get that just being there is an intervention, and meaningful. But I dunno. Instead of being proud of what I had done, I just felt upset that I couldn't do more.