I have given GATE exam once and it is my second trial. I have asked to many seniors about the books. But now I am senior myself and I have tested so many books. According to me. Every books is good in a specific subject. No book is the best for all subject. But if we talk about a single book then I would like to recommend Nodia publications as a perfects books that has good content in most subjects.
And books are not sufficient to crack this exam other type of stuffs like GATE previous years papers and GATE mock test papers are also required. Which are available on many websites like Nodia.