WOW..this is always a hard one. It seems like the book I'm studying at the moment is the most powerful book with the message that is needed most to mankind. So that changes nearly every month. I guess the way I would answer this is..."for what reason do you ask?". If your looking for a message to the lost world as a whole it is hard not to say John and Romans. But these books do not set God as God. They set Christ as God and mans need of salvation.
For those that do not believe in God or who do understand who God is, we must look to Genesis and Exodus maybe Leviticus and Daniel. Genesis (and Job for that matter) tells us there is a God, Exodus tells us that God has laws. Leviticus tells us that God is holy, Daniel tells us God is in control of all things. Now I know some will say you get this picture in Romans. But Romans does not set it in stone as well as these OT books. Romans pulls from these books to set doctrine in the OT. Its always best to go to the source.
If your are talking to a Jew that believes in God, the best book is Hebrews. Hebrews has more doctrine in it then Romans.
If you grow up in a "Churched" arena, know and believeing in God yet not taking Christ as your Lord Galatians maybe the best book.
If you are a believer you always go to the Psalms when you are dealling with something in your life. Psalms is the believes baby blanket and in this light is the most important book.
Now...i saved one that very well may be the best book for ALL mankind for last. This is for those that do not believe in God and those that believe but have lost their love for him. This book does not have the gospel, but it points you to the gospel. This book does not prove God, yet it tells you God is the answer. No doctrine is set in this book by itself. Little talk is about God in the book. This book will depress you. It will take all your dreams, ideas and desires and crush them. Your thoughts will be as the writers was..”maybe I should take my own life”. This book will make you think of death…your death. You will die someday you better be ready for it. This book is about life on earth without God. Life "under the sun" is NOTHING.
If I could have one blessing given to me, it would be to place Ecclesiastes in every persons hand with full understanding of the book. Life is nothing..nothing NOTHING unless you have God.
Chapter 12...
Remember also your Creator in the days of your youth, before the evil days come and the years draw near of which you will say, "I have no pleasure in them"; 2before the sun and the light and the moon and the stars are darkened and the clouds return after the rain, 3in the day when the keepers of the house tremble, and the strong men are bent, and the grinders cease because they are few, and those who look through the windows are dimmed, 4and the doors on the street are shut--when the sound of the grinding is low, and one rises up at the sound of a bird, and all the daughters of song are brought low-- 5they are afraid also of what is high, and terrors are in the way; the almond tree blossoms, the grasshopper drags itself along, and desire fails, because man is going to his eternal home, and the mourners go about the streets-- 6before the silver cord is snapped, or the golden bowl is broken, or the pitcher is shattered at the fountain, or the wheel broken at the cistern, 7and the dust returns to the earth as it was, and the spirit returns to God who gave it. 8Vanity of vanities, says the Preacher; all is vanity.
9Besides being wise, the Preacher also taught the people knowledge, weighing and studying and arranging many proverbs with great care. 10The Preacher sought to find words of delight, and uprightly he wrote words of truth.
11The words of the wise are like goads, and like nails firmly fixed are the collected sayings; they are given by one Shepherd. 12My son, beware of anything beyond these. Of making many books there is no end, and much study is a weariness of the flesh.
*****HERE IS THE KEY FOLKS….
13The end of the matter; all has been heard. Fear God and keep his commandments, for this is the whole duty of man. 14For God will bring every deed into judgment, with every secret thing, whether good or evil.
In Christ..James