I am pretty sure that's not what it means. But a clearer way to phrase it would have been "favorite movie adapted from a book", guys. Clarity!
Although I will grant you that movie adaptations are complicated territory, and it might be more about a book that was dear to you and turned into a movie, and just your general thoughts on that. But in that case the word "favorite" becomes weird. In general this is a weird question.
The actual answer to this question is Scott Pilgrim vs. The World, which I think is the most successful adapation around, but I already cheated once with comics, so I'm gonna do a legit book this time. And in fact I'm gonna go with a book AND movie that I have already mentioned.
Maurice, by E. M. Forster, adapted by Ismail Merchant & James Ivory
Merchant-Ivory, as they are called, are pretty brilliant in general with their Forster adaptations, of which they did three. This one is easily the best one, and not because I suspect it is also the best book of the three (though I admit I have not read any other Forster yet). It loses some things, of course, but enhances others, and I think it was played with a great subtle honesty that captures the real essence of the book. This is an empowering movie for gay men, which is a rare bird indeed, and it is sexy as hell, and it was made in goddamn 1987 featuring a very young and smooth-faced Hugh Grant, and an unbelievably gorgeous tinykins Rupert Graves (pictured, left). Also James Wilby (pictured, right), but most people don't know who he is on account of his not appearing in A) every romantic comedy ever B) an outrageously popular BBC miniseries.
True story: I once read a review of this movie on Amazon dot com from a woman who said that before she saw this movie, she had a low opinion of homosexuals and had no idea that their love could be meaningful/real. She claimed with fairly heartfelt words that this movie legitimately changed her perspective. If that's not success I don't know what is.