Zelazny should be on the list, either with Lord of Light or Eye of Cat.
Lord of Light is one of my favorite books actually. Can't really explain why, but it's got the right mix of pacing, pseudo-mythicism, and twist to it. His Amber series is an easy read and gets a bit silly/strange towards the end, but is worth reading once just for the interesting concept and approach to the world. I've always liked Zelazny for his "interesting" story worlds.
I'd also recommend Niven as an author. Ringworld is an obvious choice, but another interesting set is his Integral Trees books (warning: It's more hard sci-fi than most like). Niven also has a collection of books broadly called "known space" with a somewhat common storyline. Several of those books are pretty good reads. I recently had "Protector" lent to me and couldn't figure out how I'd missed reading it before. Very good (and filled in some gaps of info I'd run into in other stories). He's got a collection (called "Tales of Known Space", go figure), that's a pretty good set of stories set in that line (or get the updated "Three Books of" instead of "Tales of" I suppose). I also recall a set of short stories that dealt with the use of transfer booths and how they could make a mess of law enforcement, but can't remember the name.
Heinlein's more hit or miss IMO. You have to remember that he did a whole run of juvenile books (intended for young teens), but later moved into more serious and adult themes. Good news is you can hit a whole gambit of possibilities. Bad news is that you might not know what you're getting based on the name on the cover. Aside form those previously mentioned though, try: Time enough for Love, The Moon is a Harsh Mistress, and Methuselah's Children.
If you're into powered combat armor, but don't like the pro-military slant of Starship Troopers, try Joe Haldeman's "Forever War". He puts the **** in "war is hell", and examines some interesting aspects of fighting an inter-galactic war when time dilation is in effect from the technological, sociological, and psychological perspectives.
If you like that combat armor thing, plus giant bugs, interesting side stories, and a pretty cool (but kinda predictable) twist, try John Steakley's "Armor". Fun fun book. Dark, I suppose, but engrossing. The book is split into two story lines. First time you read it through, the "main" story is something you just want to get through to read about the parts with the combat armor (first quarter of the book is one combat event, then it breaks jarringly to another story, then dribs and drabs the backstory to you). It almost gets in the way. Second time you read it though, you appreciate more of it though. Another book I really like and will pick up and re-read periodically.
So so many more, but I'll stop there. Edited, Sep 13th 2012 7:30pm by gbaji