"Best" is really subjective to taste, but I'll lend my thoughts (even though they've already been pretty much already mentioned lol).
To go along with a point that was mentioned, for me at least, I know that my definition of "the best" book has varied as I got older.
When I was three years old, at least once a year until I was old enough to read them myself, my dad would read me the Lord of the Rings trilogy. His father read it to him, and I will read it to my child if I ever have one. That takes the childhood favorite category, although I still consider it one of the best I've ever read.
In my high school years I went off on a reading binge and started reading anything and everything I could get my grubby hands on.
I think out of that era, I'd have to say Les Miserables took the cake on that one. And yes, the unabridged version. At the time it was like nothing I had ever read and I loved it. Along with that, I would have to say the original Dragonlance Chronicles (Autumn Twilight/Winter night/Spring Dawning/Summer Flame). Another high school fave was The Matarese Circle by Robert Ludlum.
And yes, I'll admit it too, Elantris/Mistborn top the list for right now.