A very belated happy birthday
@Jon Snow.
This was very difficult to choose for some categories and I think this list is more than a little flexible (not just because of hitherto unread novels). They are in no particular order.
Young Adult (series or trilogy or stand-alone):
1. The Space Trilogy by C. S. Lewis
2. The BFG by Roald Dahl
3. The Secret Garden by Frances Hodgson Burnett (not really fantasy, but it was certainly magical)
4. Jupiter Novels by Charles Sheffield
5. The Stainless Steel Rat by Harry Harrison
Fantasy (series):
1. Poison Elves by Drew Hayes
2. John Constantine, Hellblazer by various writers (but especially the Jamie Delano & Garth Ennis runs)
3. Thieves and Kings by Mark Oakley
4. Bone by Jeff Smith
5. Sandman by Neil Gaiman
6. Cerebus by Dave Sim
Fantasy (trilogy):
1. Lord of the Rings by J. R. R. Tolkien
2. Farseer by Robin Hobb
3. Damiano by R. A. MacAvoy (this trilogy has grown on me since I read it)
4. The Fionovar Tapestry by Guy Gavriel Kay
5. Jon Shannow by David Gemmell
Fantasy (stand-alone):
1. The Master and the Margarita by Mikhail Bulgakov
2. Mythago Wood by Robert Holdstock
3. To Reign in Hell by Steven Brust
4. Till We Have Faces by C. S. Lewis
5. Perdido Street Station OR The Scar by China Mieville
Science Fiction (series):
1. Culture by Iain M. Banks
2. Hyperion Cantos by Dan Simmons
3. Revelation Space by Alastair Reynolds
4. Childe Cycle by Gordon R. Dickson
5. Alliance-Union by C. J. Cherryh
Science Fiction (trilogy):
1. Takeshi Kovacs by Richard K. Morgan
2. Helliconia by Brian Aldiss
Science Fiction (stand-alone):
1. A Canticle for Leibowitz by Walter M. Miller, Jr.
2. Excession OR Surface Detail (both are independent installments in a series) OR Feersum Endjinn by Iain M. Banks
3. Radix OR Last Legends of Earth by A. A. Attanasio
4. The Dispossessed by Ursula K. Le Guin
5. Inverted World by Christopher Priest
Science Fiction/Fantasy (series or trilogy or stand-alone):
1. Miracleman by Alan Moore & Neil Gaiman
2. The Book of the New Sun by Gene Wolfe
3. The Dying Earth by Jack Vance (with Cugel's Saga being the favourite installment)
4. Wraeththu Chronicles by Storm Constantine
5. Aeon Flux Shorts by Tony Cheung (animated)
Duologies (science fiction or fantasy):
1. Soldier in the Mist/Soldier of Arete by Gene Wolfe
2. Sailing to Sarantium/Lord of Emperors by Guy Gavriel Kay
3. A Fire Upon the Deep/A Deepness in the Sky by Vernor Vinge (independent installments)
4. Vurt/Pollen by Jeff Noon
5. The Infinity Concerto/The Serpent Mage by Greg Bear
6. The Sparrow/Children of God by Mary Doria Russell
7. The Peace War/Marooned in Realtime by Vernor Vinge (with a short story set between the novels)
8. The Risen Empire/The Killing of Worlds by Scott Westerfeld
9. Ilium/Olympos by Dan Simmons
10. Integral Trees/The Smoke Ring by Larry Niven