Same thing here. I could maybe list my top 10 or 20 but it would be impossible to put them in any order or find a single favourite. Here are some examples anyway. You can see I like sci-fi.I have read hundreds of books. Try as I might, I cannot come up with a favorite book or even a favorite author. At this point in my life, I can't even narrow it down to a short list.
"The War of the Worlds" and "The First Men in the Moon" by H. G. Wells
"Warlord of the Air" by Michael Moorcock
"Rendezvous with Rama" and "The Fountains of Paradise" by Arthur C. Clarke
"After the Flood" by P. C. Jersild
"Journey to the Center of the Earth" by Jules Verne
"His Dark Materials" (a trilogy) by Philip Pullman