Any movie from iconic series such as James Bond, Marvel or iconic directors such as Tarantino may be considered future classic. Awards winners may also be considered classic; so are the official festival posters such as Cannes, Venise, etc.
But yeah, most modern posters are pretty ugly ("floating head") so people tends to prefer byposter/side posters done by designers or having a special design; actually, Mondo posters and alternative posters-alike are doing much better than the original ones in term of classic design (the fact they are limited in number also plays a big part).
This is about the design itself and what may fall under classic image. Some may consider the price has a factor of what falls under classic. I would be wary with prices hike for modern posters.... see the recent alternate version of Tarantino movie, the prices went up like mad in a week time... and without any reason, may be "mass hysteria phenomena" .