I was mostly an AMC/OLTL fan, I didn't really get into GH until, ironically, Maurice Benard came to the show. I was a fan of his portrayal of Nico Kelly on AMC, so I wanted to see how he'd fare once he came to GH. I say ironically because he got me into the show, I see the despicable character he's playing, now he's the lone aspect I hate about GH (that and the mob). This bi-polar/abuse/downward spiral storyline he's in is fascinating to watch, though, I will admit.
So for me...basically when the Cassadines came on the scene. Baby Lulu needed something that only a blood relative could give, Lucky wasn't a match, so Laura had to reveal when she was trapped on Cassadine island she had Stavros's son, Nikolas. Nikolas was a match, he saved Lulu, now the Cassadines were a permanent fixture going forward. That whole storyline, Nikolas and Stefan (Nikolas's uncle and guardian), was great.
Second would be Helena's introduction. Well, introduction as played by Constance Towers I should clarify.