The 1998 Locus Poll & Survey ballot, available with the February 1998 issue, includes categories for Best All-time SF Novel and Best All-time Fantasy Novel. (You can get the issue by subscribing or ordering now.) These three pages show the best all-time novel results from 1975 and 1987.
1) The Lord of the Rings, J. R. R. Tolkien (1955)
2) The Hobbit, J. R. R. Tolkien (1937)
3) A Wizard of Earthsea, Ursula K. Le Guin (1968)
4) The Shadow of the Torturer, Gene Wolfe (1980)
5) The Last Unicorn, Peter S. Beagle (1968)
6) The Once and Future King, T. H. White (1958)
7) Nine Princes in Amber, Roger Zelazny (1970)
8) The Chronicles of Thomas Covenant the Unbeliever, Stephen R. Donaldson (1977)
9) Dragonflight, Anne McCaffrey (1968)
10) Little, Big, John Crowley (1981)
11) Alice in Wonderland, Lewis Carroll (1865)
12) The Gormenghast Trilogy, Mervyn Peake (1950)
13) The Riddlemaster of Hed, Patricia A. McKillip (1976)
14) The Incompleat Enchanter, Fletcher Pratt & L. Sprague de Camp (1941)
15) Watership Down, Richard Adams (1972)
16) The Dying Earth, Jack Vance (1950)
17) Glory Road, Robert A. Heinlein (1963)
18) A Spell for Chameleon, Piers Anthony (1977)
19) Dracula, Bram Stoker (1897)
20) The Wizard of Oz, L. Frank Baum (1900)
21) Silverlock, John Myers Myers (1949)
22) Something Wicked This Way Comes, Ray Bradbury (1962)
23*) The White Dragon, Anne McCaffrey (1978)
23*) The Stand, Stephen King (1978)
25) Lord Valentine's Castle, Robert Silverberg (1980)
26) The Chronicles of Narnia, C. S. Lewis (1950)
27) The Shining, Stephen King (1977)
28) Conjure Wife, Fritz Leiber (1953)
29*) Deryni Rising, Katherine Kurtz (1970)
29*) The Worm Ouroboros, E. R. Eddison (1922)
31) Witch World, Andre Norton (1963)
32) Salem's Lot, Stephen King (1975)
33) A Wrinkle in Time, Madeleine L'Engle (1962)
