It depends. Peter F. Hamilton's The Reality Dysfunction is now considered something of a classic: it's been out for nearly thirteen years, it's absolutely festooned with reccomendations (there's three or four pages of rave reviews in the front cover, including from authors like Iain Banks, Colin Greenland, Stephen Baxter, Gregory Benford and David Langford) and it's jam-packed with great ideas set in an imaginative and colourful universe with a page-turning plot told extremely well. I'd recommend it to a newbie without a problem. It may be 1,200 pages long in paperback but those pages will fly past a lot quicker than books half its size. Hamilton is simply a damn fine storyteller, which in SF is rarer than in fantasy.
So that would be my reccomendation, especially if you like things like Battlestar Galactica. For other types of SF, Richard Morgan's Altered Carbon is well-recommended for cyberpunk-style detective fiction and Christopher Priest's The Prestige is an excellent example of character-driven SF set in the past.
If someone had asked us to recommend a detective novel, why would we suggest one by Dorothy L Sayers? I still don't get this insistence on so-called "classic" sf novels. They're only relevant with the history of the genre. and, let's face it, bar a few exceptions most early sf writers were poor stylists.
Had you read Canterbury Tales prior to reading Hyperion? Did not having read it spoil your enjoyment of the book? What about all the references in it to Keats' poetry? See, if you can read a sf novel and miss all the literary references and inspirations, why can't someone else read it without prior knowledge of the sf references and inspirations?
Ian, is that really what we are talking about here? Old books or new ones only? I thought we were just trying ot come up with a good list. I personally would put more older books on that list, but some new ones too.
but for a first dip into SF, I would go for an anthology. several writers with varying styles gives a much broader picture of the SF from any particular time.