If you had to choose just one, it's this one ...
 The Player's Handbook contains everything you need to play Dungeons and Dragons 5th Edition.
 Create your character step by step among the many races and classes, understand the combat system, the different spells as well as the peculiarities of your world and know the rules and operation of Dungeons and Dragons 5th Edition inside out.
 This book is a "must" for both players and game masters!