This happened to be an interesting topic, and I do appreciate all the comments that the previous post generated. In preparing to write about the top 20 science fiction novels I'd choose, I thought I'd go over the earlier listing and discuss why I agree (and disagree) with those choices. Please feel free to chime in and criticize my criticisms, if you'd like.
1. The Lord of the Rings (1954/55) - Technically not SF, but fantasy. However, the two genres are very much related to each other, and some authors like C.J. Cherryh bounce between the two fields. While I'd prefer not to have LOTR on a list of the best SF novels, the book is so influential that you can't really not have it on the list.
2. Time Enough for Love (1973) - Enjoyable book, especially for "The Notebooks of Lazarus Long," but I wouldn't rate it #2 and, in fact, I'd drop it from the "top 20" altogether. (This is a book I've reread recently as well.)
3. The Martian Chronicles (1950) - Another book I've reread in the past few months. Influential book? Absolutely. Worthy of being in the "top 20?" Not a chance. And the writing is very dated at this time.
4. The Moon is a Harsh Mistress (1966) - Sad to say, I haven't had the chance to reread this one in a long time. I remember Heinlein's use of a Russian "accent" to be off-putting at the beginning, but it grew on me as I continued reading the book. I'm not sure I can remember enough about the book to say whether it's worthy of a "top 20" honor, but I'll be conservative and say "no." I expect I'll need "room" on the list for other books.
5. Fahrenheit 451 (1953) - Another book I haven't read in a long time. Another book that, while influential and speaking on an important topic (censorship), I'll also pass on.
6. Ender's Game (1985) - Worthy of a "top 20" listing; worthy of being made into a major motion picture. Until about 10 years ago or so, making this novel into a movie would have been extremely difficult (a lot of wirework at the very least); in today's realistic CGI world, the "battle scenes" that are at the heart of the novel should be a breeze for today's filmmakers to do. A movie based on this book should pull in "Harry Potter" numbers at the theaters.
7. Second Foundation (1953) / 8. Foundation (1951) / 12. Foundation and Empire - One of the things that struck me about this list of "top 20" SF novels is that the books seemed to be picked either because the plot was based on a strong conceptual idea or because the book was a sentimental favorite. The Foundation series is another one of the former. I had originally read the Foundation trilogy when I was around 20 years old; about a year ago, I reread "Foundation." Such terrible writing. Isaac Asimov became a great writer, but the first book is not representative of his eventual skill. However, once again, yes, an incredibly influential series. (George Lucas' planet of "Coruscant" was obviously based on Isaac Asimov's "Trantor.") Worthy of a "top 20" honor? No, not today.
9. Dune (1965) - This is a book that I often reread (actually, I'll go through all of Frank's Dune novels in one go, from start to finish). The writing remains fresh, even though the book was written in the early 60s. Most definitely top 20. To be honest, I'd probably rate it #2, behind LOTR.
10. Starship Troopers (1959) - Another book I've reread in the past year. As a Heinlein "juvenile," it's better than most. It's worthy of a "top 20," but I'd place it in the last quarter.
11. Rendezvous With Rama (1973) - Haven't read it in a long time, but it's worthy of a "top 20." I read this for the first time when I was in my early teens and, well... it made a great impression on me (as did several other of Arthur Clarke's novels).
13. Pet Sematary (1983) / 14. Farnham's Freehold (1965) - I haven't read either of these books, so in all fairness I can't really say whether they should belong in such a list; however, I'm going to exclude them from my own listing.
15. 2001: A Space Odyssey (1968) - Read the short story (The Sentinel), read the novel (several times), watched the movie (don't know how many times). But is it really worthy of a "top 20" in today's era? I don't think so.
16. Stranger in a Strange Land (1961) - I consider this novel to be the last and best of Heinlein's juveniles (although it's obviously not one of the "official" Scribner juveniles). Although certain aspects of the writing have become somewhat dated (and Heinlein's various discussions about Islam are both accurate and completely off the deep end in equal measure), it remains, IMO, one of Heinlein's best works. I would rate it higher than either of the other Heinlein books discussed above.
17. Speaker for the Dead (1986) - It's been a while since I've read this novel; I think it resonates with people the most for the semi-spiritual aspect, the "speaking" for the dead. Although it's a direct sequel to Ender's Game, it's written in a completely different frame of mind. (Imagine George Lucas following up Star Wars with, say, Solaris, using the same characters.) I'm not sure whether it's still "top 20" material, but I'll give it the benefit of the doubt and say "yes."
18. Have Space Suit - Will Travel (1958) - No. Another Heinlein juvenile. OK plot. Purely in this list for sentimental reasons. Nice to read, but not anywhere close to "top 20" material.
19. Childhood’s End (1953) - Yes. It's been ages since I've read this, but some parts of this novel have remained vivid memories.
20. Glory Road (1963) - No. Haven't read it since '83 or so. Who were these guys who voted for this novel trying to kid? It's OK, but it's nowhere near "top 20" material.