As for the worst, it's the opposite of that: when they ditch the original movie poster for the dreaded "floating heads" of a key star or two or even worse throw some cheap-ass generic thing on there that doesn't even clue you in to what movie it's supposed to be. For the first, check out
Open Range. It had a pretty cool poster, but obviously some genius in the marketing department decided a stock shot of some galloping horses being haunted by the floating heads of the film's three biggest stars would be a whale of a lot better. What the fart?
Or a modern classic like
An American Werewolf in London that had a memorable poster only to be replaced by a floating head of a special effect against a generic blue filtered London skyline. THAT was a good idea.
Or where they ALMOST decide to use the original poster concept...but then chicken out and screw it up by imposing floating heads, like for David Fincher's
Zodiac. How totally unnecessary.
As for an example of a cheap-ass stock photo from some service that gives you no clue what it even is other than in it's most generic sense, I give you the DVD for the 1956 Western
The Proud Ones. Now the people who might actually be interested in purchasing or renting this movie are exactly the same people who would appreciate knowing that Robert Ryan, Virginia Mayo, Walter Brennan and Jeffrey Hunter are in it. Instead they've tried to keep it totally generic, I assume so that a thirteen-year-old who really liked the
3:10 to Yuma re-make and thinks Kurt Russell totally should have gotten the Oscar for
Tombstone might accidentally rent
The Proud Ones...which almost surely they will hate. What a great trick! Those who want to watch it may pass it up, and those who have no use for it may pick it up on a whim. Terrific.