Which is the best and most useful book for competitive English grammar?

It all depends on the type of exam you are taking. If you are preparing for SSC/Bank then go for corrective grammar by Ajay Kr. Singh. It is very easy to understand. For practice and command buy, A mirror of common error by Ashok Kr Singh and English is easy by B.Sc publication. If you are preparin

1. English Grammar & Composition Very Useful for All Competitive Examinations by SC Gupta 2. Objective English by Edgar Thorp

You can go through Oxford or Cambridge grammar books. Nowadays many are there. You can look at Scholastic range of grammar books also. It gives vast variety of grammar exercises.