I am going to try to pick books that people have not already picked.
A Wrinkle in Time-Madeline L'Engle
This is geared for younger readers but it really started a lot of kids relating to science fiction and I still love her books
Any of the Anita Blake books by Laurel Hamilton.
I wasn't a big alternate reality fan but these are great, I will never think of St. Louis the same way **grin** Vampires, zombies, and ghouls...oh my!!!
The Talisman by Peter Straub and Steven King.
I love The Stand also but I figure a couple people mentioned it and this one is King does fantasy...
The Folk of the Air by Peter Beagle.
SCA-type folk, faeries and San Francisco...sounds cliche..is actually really a good book.
The House of the Wolf Trilogy by MK Wren.
An obscure trilogy about the sociological aspects of revolution...and a pretty good read anyway
Tea with the Black Dragon by RA McAvoy.
A small fantasy novel...the interesting thing about it for many is that it is so VERY dated as far as the use of computers but it is a great read...besides not many fantasy heroines are post-menopausal Irish fiddle players.
The Time of the Dark series by Susan Cooper.
Again for younger readers, although I still like them. Welsh based adventures of two sets of kids who meet partway through the series.
Atlas Shrugged-Ayn Rand
Well okay, maybe not strictly a science fiction/fantasy book but I think an arguement could be made to include it.
The Darkover Series by Marion Zimmer Bradley.
You know I love these books and not only for what themselves but for the amazing amount of other writers they have influenced or who have begun their professional careers by writing short stories for the anthologies (Mercedes Lacky, Jennifer Robeson among others)
the Honor Harrington
series by David Weber
Okay I know I am going take it on the chin for these from many people but I love them...old style science fiction space opera...it is a formula that worked for a long time