The "Best" could be interpreted in so many ways. The Bible, as a book, really does have it all. War, drama, supernatural, sacrifice, court intrigue, magic, even sex and violence. Still, there's a special place in my heart for Jonathan Livingstone Seagull.
For me, it is The Godfather.
The journey of Michael Corleone from a straight-laced soldier to an avenging murderer to a Don of Italian-American mafia empire, I enjoyed it very much.
I also find the backstory of Vito Corleone interesting, from a humble husband to a Don after offing the previous Don.
The intrigue, interactions between Don (Especially Vito) with his underlings and/or fellow Italian-American, as well as the web of assassinations following the drug-trade, very fascinating!
The Communist manifesto is the best book I've ever read. It is how every society should be run.
My favorite would be the Necroscope book by Brian Lumely. The vampires in those books are the way they should be, smart, evil, dark, and perverted. They don’t shimmer in the sunlight and there isn’t a ridiculous love story. Other than those I really liked the Wheel of Time series but just couldn’t finish it. I’m currently reading A Pattern of Shadow and Light by Melissa McPhail which I’m liking more than the Wheel of Time.
Best book is hard to say. Different books have different purposes. One of the best books I ever read is The Book of Five Rings by Musashi Miyamoto. I would recommend it to anyone who studies martial arts.