My first thought, when I saw this thread, was Brad and Colleen. I think it was so shortsighted to kill both of them off. After reading through all of the replies, I agree with Ji-Minh, Blade, and John.
Digging way back, I still miss Ryan. I started watching Y&R in 1991, so he was part of my introduction to the show. I loved his story with Nina and Victoria, his friendship with Neil (shirtless basketball...), and, later, his storyline with Tricia. Which, also brings up all of the Dennisons - Tricia, Megan, and Tony. I've watched some of the old episodes from that time on YouTube, and Tricia was such a creepy crazy person, Tony was a great "bad guy with the heart of gold" (remember his relationship with Cassie?).
Oh, and killing off Cassie was kind of dumb, too.