Stewart is a big one for me. Celebrities passing at an old age usually isn't much of a thing, especially if they were suffering from mental decline. But Stewart became an icon. There was only one Kirk, so he became the only Picard. It just felt like the world wasn't ready to lose Stewart yet. Avery Brooks better live to be 140! He is the darn Emissary and all. Then the captains kind of drop from there.
Koenig was tough because he was one of my favorite villains of all time from B5. So subtle, so effective, so well played, the one-off character became a much more important one.