Looking back, I see I came to SF quite late compared to most of you, and indeed am still playing catchup.
The Hitchhiker's Guide to the Galaxy
I'm sure this was first. I read fantasy almost exclusively as a teenager, but my girlfriend persuaded me to read this and I loved it. I haven't seen the movie or heard the radio show or re-read it since, so I don't know how I'd enjoy it now. But I loved it twenty years ago, I was around 15. See how late I was to the party? To see some of you were reading Asimov at 9 years old before I was even born.. is humbling.
The Foundation Trilogy (Foundation, #1-3)
I went back to reading fantasy through college, spent my four years through university reading only Computer Science textbooks, and when I started my first job and my boss discovered I'd never read *any* SF, he put me on a crash course that began with Ringworld, The Foundation Trilogy, and Iain M. Banks. I remember enjoying the Foundation books at the time but today, 15 years later, remember *nothing* of them. It's the only Asimov I've ever read, so far.
1984
Brave New World
Slaughterhouse-Five
Fahrenheit 451
Stranger in a Strange Land
These I read in quick succession, possibly even the same year (quite some time after starting with Foundation). I think I found some "classics of SF" list somewhere and decided to hit it up. 1984 would be my favourite novel of these five, though I don't really think of it as SF. It's on my literature shelf at home, for example. Brave New World and Slaughterhouse-Five, looking back I think I must have read them too quickly as I don't remember them making much of an impact on me. Many books listed as "classics" get read quickly by young readers looking for that magic upper-cut, that impactful experience, and sadly much of these great works is lost on such a first contact. Perhaps one day I will re-read them. Fahrenheit 451 I liked and have re-read since, I think. It fires up my passion for libraries and equal access for all to physical books that cannot be owned, edited, retracted, or limited in any way by the corporations of this world. Stranger.. I barely managed to finish. I can only assume it spoke to an earlier youth with more resonance than my 28 year old self in 2008.
Dune
I read Dune one hot summer while camped out in a tent, fishing. I had to ration my water so it would last my stay, and I remember the sun melting some of the glue that reinforced the stitched seams of my tent. I don't remember much about the book other than I really enjouyed it. I would re-read Dune. I would try a sequel, or two, maybe..
Ender's Game
Still on my "classics" binge, I read Ender's Game. I didn't like it. I might have enjoyed it more as a teenager and certainly if I'd not known anything about the author.
Do Androids Dream of Electric Sheep?
Hyperion
I think these were the last two of my classics read-through, both read within the last five years. I enjoyed them both, I was reading literature on the side (inc. some Keats) with Hyperion and the two fused very well. I woudl re-read Hyperion, and despite the controversial views, I would try the sequel(s) one day. Do Androids Dream was the first Phillip K. Dick I read. I like Blade Runner as a movie, but am not so zealous about it that I can't read DADOES as the book it is. I liked it. By today's standards, I agree it is perhaps a little dry. Similar themes might be more aptly explored in more recently published works.
I, Robot
2001: A Space Odyssey
I haven't read these yet.
Looking back, I realise what a terrible memory I have. I read Dune in the last five years and remember very little about it. I read the Foundation books fifteen years ago and remember absolutely nothing. I never used to be one for re-reads, but as time goes by and I forget more and more of what I've read, re-reads begin to look more possible.
Of the list above, I would like to re-read the Foundation books and Dune - but there is so much I still want to read that I haven't, that isn't likely to happen any time soon.
My experience of that "classics" list was that mostly they were disappointing. I fell for the trap I think many do - the criteria by which a book is considered classic is unlikely to match the criteria by which you yourself rate books so many years after those classics are established as such. And those who discovered them back when they were in vogue and earning their classic status can't objectively see them today without the tinted spectacles of nostalgia.
What Top 10 would you give to SF newcomers today? Would you still recommend the ten listed above?