+1 LJ
Douglas Adams is absolutely wonderful. I also recommend Eoin Colfer's extension of the series.

THE CHRONICLES OF THOMAS COVENANT THE UNBELIEVER by Stephen R. Donaldson stands out for me.

Maple, my dad and sister both loved those books, I tried I really did, but I just couldn't get into it, maybe I tried too young (think I was about 14)

Ursula Le Guin stands out for me as missing from the above:
The Lathe of Heaven
The Dispossessed
The Left Hand of Darkness
City of Illusions
Rocannon's World
Planet of Exile

@sean - I hear you.

Donaldson went out of his way to make the protagonist as unpleasant as possible. Thomas Covenant is one bitter piece of work. I read the books when I was 19. I find them to be kind of simplistic now, but Covenant's "fuck you" personality is refreshing compared to faggy Frodo or Harry Potter.

Adams ripped off his work from The Cyberiad. The similarities are too close to be wholly original.

Adams was a brilliant writer. I have to say I enjoyed the Dirk Gently novels more than the Hitchiker ones. Not sure they qualify as SF though.

Surprisingly, to me, I also enjoyed Wyndham's Day of the Triffids. Not what I was expecting it to be.

The Chronicles of thomas, didn't get into the 1st pages... *shrugs*
Ursula, read 'Left Hand of' - was just 'ok.

In the fantasy genre: The Wheel of Time by Robert Jordan (and now by Brandon Sanderson) series is one of my favs. Although I refuse to buy the latest as it's not available in digital format. I also quite enjoy the Dresden File series by Jim Butcher. Sword of Truth series by Terry Goodkind is excellent and is a thinly veiled commentary on capitalism vs communism, but is enjoyable just the same. For purely SciFi Battlefield Earth by L Ron Hubbard and Foundation series by Isaac Asimov.

Terry Pratchett - not exactly science fiction but definitely fantasy and very funny.

Lois McMaster Bujold – the Miles Vorkosigan series

Asimov – Foundation series

Eric von Lustbader – wrote many good (and some not so good) stories – try the Sunset Warrior

Larry Niven and Jerry Pournelle – agree that the Mote in God’s Eye was good, try Footfall by the same authors.

And if you like Halderman, then I suspect you’ll like Bernard Cornwell. BC writes historical fiction, the Sharpe series and Thomas the Archer plus others.

@TrustMe

Yeah, Dresden Files are a lot of fun. Too bad the TV series was a pile of garbage.

Iain M Banks is my favorite (hence the user name). Consider Phlebas is a good place to start (I did) although The Player of Games is the easiest read (and would appeal to a Dip audience) and Use of Weapons is my favorite.
Ditto the John Scalzi comment above (Old Man's War is a good one for a teenage nephew/niece if you are looking for a last minute Xmas gift ...)
If you enjoy John Wyndham's Day of the Triffids I would recommend The Chrysalids (called re-birth in the US I think) by the same author, slightly different set up than most of his but actually resonates the most.
Digressing to fantasy then George RR Martin's Song of Ice and Fire series (Game of Thrones is the first) takes some beating although it would be nice to know he will finish it at some point, if you want to read a series straight through give him another decade before you start).

ARGH - Please No Fantasy !! (start your own post - can't stand that 'style')
;-)

Well, how would you classify Stirling's "Change" series, Science Fiction or Fantasy?

can't believe Octavia Butler hasn't been mentioned -- "parable of the sower", "parable of the talents" and the incredible "fledgling" are standouts, for more more classic sci fi, there's also the lillith's brood series. anything by her is great, though.

a newer author who i quite like is Nalo Hopkinson. just finished "midgnight robber" and rad "brown girl in the ring" a bit ago. about to read a book by Nnedi Okorafor, "he who fears death." also definitely second Samuel R Delaney -- for something very long and mind blowing, try the Neveryon series.

To give this topic some new attention (so i can have some more reading suggestions :))

I am a big fan of david Brin, Jack Vance and Michael Nijk.
Some new series i''d really liked are The Lost Fleet, from Jack Campbell and Humanity's Fire from Michael Cobley

Van Vogt ! Brin is kewl.

James P.Hogan's 'Giants' series has to be some of the best hard sci-fi ever written.

@Dharmaton

I've read a little of Van Vogt, but it's a little weird for my taste.

If you're a fan of distopias, Dhalgren is by far the best.

Fair warning: It is very sexually graphic.

I'd also recommend R.U.R by Karl Capek. The first work to use the word "robot." Really a must for a SF buff.

Also, by Capek, I recommend War with the Newts. It's in that period before SF was really SF, but still one of my favorites. I've yet to meet anyone else to have read it.

@Dhar

Do you hate all fantasy, or just prefer really hard fantasy, such as LotR?

I would highly recommend Lord of Light, by Roger Zelazny, to just about anyone. Although I don't know if it's sci-fi, so much as it's philosophy, written using the idiom of science fiction.

Neuromancer is another good one. The definitive book for the Cyber-punk genera of SF.

Neuromancer is damn good, but I would be hard pressed to say it is more definitive than Snow Crash or Diamond Age.

Well, Neuromancer is a decade older : )