Personally I find Aika's MM graduation to be one of the most heartbreaking, but Akarin's grad ceremony from Momoclo was also fairly depressing. In Aika's case it was sad because it so wasn't what she wanted and it felt like she never really got to live up to her full potential. With Akain, it was less about potential (because she was a fairly weak member, all things considered), and more just the experience that Momoclo had shared up until that point and even though the rest of the group still wanted to work towards these goals, she just wanted out and they all struggled with it. Her grad was a sob fest.
Yuzu didn't graduate from Syachi but her hiatus was probably the most upsetting for me because, like Aika, it was all totally out of her control. Normally I have like a day of mourning when a favorite idol announces graduation and then I'm fine, because ultimately I think it's my role as a fan to support a girl in whatever she wants to do in her life, even if it means I never see her again, but if a girl leaves because of illness or injury, nobody is happy or secure with the decision. In both Aika's case and Yuzu's case, however, there has been some healing just because they never really disappeared and their respective groups have reminded us constantly of their presence.