Of course, you do get occasional exceptions when either the filmmakers are directly involved or the marketing team are particularly talented and in tune with the film itself - Prometheus seems to be a good example of this.
yah I don't think you can ever say that a film adaptation is better than its source. You can say you like it better, but I don't think in any case you can say that it is better. You can only say, it was different.
Films that are better than the book: A Clockwork Orange, Fight Club, The Shawshank Redemption, The Godfather, Apocalypse Now, The Wizard of Oz, Casablanca, Children of Men, Jaws, The Prestige, There Will Be Blood, Silence of the Lambs, No Country For Old Men... and Twilight.
Easy, you just add more contrast, and make it widescreen.