Depends on where you are on your brewing journey and I doubt you could narrow it down to one.
Palmer's is good as a primer, but I much prefer Randy Mosher's Mastering Homebrew as I prefer his writing style, it delves a little deeper whilst still keeping it simple, has relevant & numerous graphical content which is really useful, has an irreverence I find enjoyable and it stresses the importance of visualisation in recipe development and critical assessment of beers in order to improve them. If I could only have one of the brewing books I currently own, this would probably be it.
Once you;re brewing AG reasonably well, Gordon Strong's Brewing Better Beer is a good read, although again his style may not be to everyone's liking and he is a bit of a traditionalist
If you're into invention and pushing the boundaries (and even if you're not), Randy Moshers Radical Brewing is excellent and full of great little anecdotes which are fun (and hopefully true!). If you want to research particular styles there are specialist books (the IPA one is a good read in its won right).
Of the ingredient series, I have all 4 and only reread Yeast and Hops. I found both Malt and Water boring and tedious
And Beer by Charles Bamforth is interesting
There are loads of others as well. Personally, I'm always on the look out for the next brewing book that might add to my knowledge and enjoyment.