Which religion gives the most happiness?

The most popular one.

C.S. Lewis's replyQ: (Audience member) "Which of the religions of the world gives to its followers the greatest happiness?"

A: (C.S.L) - "While it lasts, the religion of worshipping oneself is the best."

"I have an elderly acquaintance of about eighty*, who has lived a life of unbroken selfishness and self-admiration from the earliest years , and is, more or less, I regret to say, one of the happiest men I know. (...)

"I haven't always been a Christian. I didn't go to religion to make me happy. I always knew a bottle of port would do that.

"If you want a religion to make you feel really comfortable, I certainly don't recommend Christianity."

From C.S. Lewis - Timeless at Heart (ed Walter Hooper) - Answers to Questions on Christianity.

* I guess this was Rev Dr Frederick Walker Macran or 'Cranny' (1866-1947) - biography in All my Road Before Me - Diary of C.S. Lewis 1922-1927 edited by Walter Hooper.

h/t Bruce Charlton