Your Classics, Please

I am off later this week to the annual Jack Williamson Lecture in ever-friendly Portales, New Mexico.  This year's guests of honor will be Carrie Vaughn and Daniel Abraham.   There will also be Connie Willis, Ian Tregillis, Melinda Snodgrass, and probably a good many more.While I'm there, I'll be on a panel about science fiction classics.I have a list of science fiction classics to wave around, naturally, but I don't want to leave anything out.What should be on my list?  What treasures of...