I guess by default it can't be worse, regarding Classic Five-O losses, since all but one of the main characters are already gone.

It has been 53 1/2 years since Five-O started, and none of the characters except Danny Williams were young, so it was inevitable that almost everyone would be gone by now.

The most surprising thing to me was that George Gaines, who played a grandfather on Five-O in1970, lived until 2016!