I'm not an artist and have positively no sense of design, but Silence of the Lambs and Jaws seem to me to be downright outstanding, each for different reasons. (Many editions of Benchley's book use the same basic image on the cover; not sure whether it was used on the book first or was created for the film.)
I always admired The Exorcist's original poster:
It seems like it just takes a still from the film, but it's one of the prettiest shots. In Friedkin's recent book, I believe I recall him saying that he blocked off a whole day (well, a whole night anyway) of shooting so they could get that shot; there's more to it than meets the eye, and he felt it was important.
It occurs to me that I actually don't know a lot about the approach to poster design. It seems like these days the ones that are really good are really good, and the ones that are clunkers almost make me think I could have done better. (The poster for a horrible recent film called One for the Money is simply Katie Heigl wearing tight jeans. It's not a movie poster. It's a poster for how taut her ass is. Wait, actually, I'm okay with that poster. Carry on.)
Seems like good posters tend to stick in the mind. I'm not sure how that effect gets achieved, actually, but I can tell you that I don't know what the poster looks like for some of my favorite movies ever, whereas I'll never forget ones from movies that were horrible-but-nonetheless-had-cool-posters. Also it's cool when a poster can be evocative before you've seen the film, and then take on a whole new resonance or meaning after you've seen the film. (Silence of the Lambs and Cabin in the Woods are like that.)