I left out Jules Verne and H.G.Wells on purpose because I think their writings would be a bit awkward / tedious for a modern audience.
Having said this I Should have included John Wyndham (and I will in Part 2 :-)
I'll have to re-read "Childhood's End" because I can't remember the "feel" of it in enough detail to make a decision.
I've never heard of "Wreck of the River of Stars" but the reviews are good on Amazon and I've just added it to my "Wish List"
My "favorite" of RAH, ACC, IA, and AN depended on which I was reading at the time, and I frequently had several by different authors going at once. They were all my favorite for different reasons.
How could you leave out Jules Verne? He invented the field.
Someone else not mentioned so far, Alfred Bester "The Demolished Man".
I would expect anyone who started out with Heinlein would be hard to please :)
One of the things that has happened with this series is that I now have a new list of books to look for when I visit the local used book store...
"Have Spacesuit Will Travel" was unique in that it was intended for younger readers and emphasized the importance of education, human values, self-learning, and getting along with each other. It also pointed out many human shortcomings.
I've read them both -- I'm currently reading "Micro" -- they are all "OK" -- but not "Must Read" (I'm a hard man to please :-)
I must admit that out of these three, "Timeline" did make me think the most...
If I had to pick only a very few books to introduce someone to science fiction, I'd choose the following (while noting that all the other books mentioned are all good reads)
For the younger audience: Heinlein's "Rocket Ship Galileo" and "Have Spacesuit Will Travel" with the rest of the Heinlein books for younger audiences held in readiness for when the bug hits :)
For intermediate readers I think Asmov's Foundation trilogy would be a good starting point.
For older readers "The Moon Is a Harsh Mistress" and perhaps "Enders Game" or maybe "The Mote in God's Eye"
In addition to the books already mentioned, Micheal Crichton's "Andromeda Strain" or "Terminal Man" might make good inclusions on the list for older readers.