Mate, you really saved some big guns for last.
2001 is the first sci-fi I read that blew me away. I'd read (I guess) "lightweight" sci-fi before, but 2001 had an immediacy and reality that appealed tremendously. I mostly mean the description of the spaceship and HAL and the planning and plotting of Discovery's mission. It has stuck firmly in my head. I often tell people 2001 is the archetypal sci-fi novel. (Which I probably shouldn't say on this forum because it seems most of you have Master's degrees in SFF and/or horror :-)
I still get goosebumps when I think of the line, "And so, for the last time, David Bowman slept."
The Dying Earth series is fantastic. To me, these stories seem to just roll along, unfolding of their own accord, apparently without a particular plot or story or quest in mind. I'm not sure I can explain this properly but take 2001 as a contrasting example. Every scene is carefully crafted to move the story along a well-defined path to a particular CONCLUSION, whereas in, say, The Eyes Of The Overworld, Cugel encounters all manner of strange and crafty creatures, the only goal being to get back home. It's like the journey is only an excuse for a bunch of excursions into the fantastic and the bizarre. I love the unpredictable nature of that, and I love the people Vance comes up with. Tremendous fun.
I can hardly go past another Dick. And similarly, I can't pass up on War of the Worlds (and now I'll have Jeff Wayne's music stuck in my head all day... not a bad thing)