Interesting question. I think my most difficult goodbye was to my dad, just a few weeks before he died. I was institutionalized and could talk with him on the phone for only 15 minutes at a time, and during our final conversation, I sensed that he didn't have much longer to live. Towards the end of our call, a recorded voice said, "You have thirty seconds left," but dad and I didn't quite know how to say our final goodbyes. Which seemed to reflect much of the awkwardness I often felt when communicating with him throughout my life (some of which was spent in Reseda, CA). In any case, we just lingered in silence during those last few seconds until the connection was automatically broken, to be followed by a dial tone -- rather like the steady tone of a heart monitoring machine that suddenly stopped recording a beating heart.

I tried to kill myself a couple of years ago, my wife found me and killed herself no one told me for over a week.

When my mom died from a long slow battle with cancer.

Oh that's so hard to put in order. They're all hard in some way. One that will never leave me without anger and regret tho is trusting someone with my 1st adult chosen dog. Psycho passive aggressively put him to sleep without telling me. So yeah, basically the murder of a heart dog.
Altho I've dear relatives and loved ones who've passed and their absences will always be felt to be robbed in such a cruel way never leaves a person. After that I had irreversible trust issues that haunt me to this day.

my st bernard x shar-pei called sags. I have never been as upset as when I lost him

My grandpa. He was my protector, someone I could talk to with out being judged and I was the only grandchild he shared a KitKat with. While every other cousins of mine got the stale cookies from the cookie jar.

My 17 year old cat when I was 21 and a week before I got married. That poor cat had been through hell, when he couldn't hurt me any more my father would torture her to get at me. I had to have her put to sleep as she had cancer. Will love that cat forever.