a must
That which is requisite, necessary, or indispensable. I know this book is a difficult read, but it's an absolute must if you enjoy literature. Taking the introductory course is a must if you want to sign up for the advanced class.
a must
A necessity; a requirement. For example, The Louvre is a must for visitors to Paris, or This book is a must for serious students of English. [Late 1800s]