So as to not dredge up a 14 year old thread on the subject I'll just state here:
By far most 'classic' SciFi series were published before my time and I hardly have enough time to read them all. So I took a little time and crawled the Internet to take a gander at a large number of "Best of all-time" lists various entities had published. Culling from those got me a short list of perhaps a hundred books/series.
Starting with the Sword of Truth (Terry Goodkind).... and I must say it was a rough start. How the ####! did this author make so many lists? His evil characters are deeply sadistic with motivations verging on the comically putrid. Even the "good guys" act alarmingly vindictive and seem perversely motivated. That, and every adult ("good guys" and bad), in every instance, come off as creepers when they have conversation with minors. Oh, and the character Samuel - how did he not get sued by the Tolkien estate for that patent ripoff of Gollum? Nope, I'm done with this series before finishing the first book.
On to the Sword of Shannara by Terry Brooks. I've seen the TV series - all two abortive seasons - but I won't hold that against the book until I read it anyway.