After 20 years of collecting posters, there have only been a handful of movies that I've never found a suitable poster to collect that has sufficiently good artwork to match the price of the poster and/or the quality of the movie. I've ranked my top 3 worst artwork to movie ratio posters. Raiders will be a bit controversial, but it makes the top 3 due to the referencing of other movies in giant bold text that is above the moderately nice Struzan artwork. This includes looking at all foreign possibilities as well. Many posters I've been able to pick up a foreign version if I didn't like the U.S. version.
Shining
Poltergeist
Raiders of the Lost Ark
There are a few others but these have the worst ratios.
