I too think it depends on what you mean by quotable. When I think of a show that spurns some of the most profound stuff or very easily used in High School yearbooks, I think RENT. Not to say it's more profound, but it certainly is more relevant I think than some of the older shows.
Of course there are other shows that can easily be quoted, sometimes w/o people even realizing it...I've grown accustomed to her face, and umm okay not many are coming to mind but I know they exist, just on the tip of my tongue.
"If there was a Mount Rushmore for Broadway scores, "West Side Story" would be front and center. It snaps, it crackles it pops! It surges with a roar, its energy and sheer life undiminished by the years" - NYPost reviewer Elisabeth Vincentelli