Which science-fiction classic do you wish someone had handed you first?

Okay so I keep seeing lists that assume you already know what you like about sci-fi, but what about the very first book? Like, is there one that basically does the welcoming committee job, where you finish it and immediately want five more? I'm curious whether the older classics still hold up as entry points or if a newer book actually does a better job easing someone in. Does the "importance" of a book matter at all when you're just starting out, or is pure readability the whole game at that stage?