[rixrex]

No really wrong answer here unless you pick one from the wrong decade, as it can be pretty much a personal thing on how you view the decades and how they affected you. Here's my choices, starting with when I began watching films, and some are definitely personal as they are the ones that defined the decades from my experiences:

50s - Tarantula, just because I saw it at the drive-in in 1959 as a brat in the back seat and was both terrified and then hooked on sci-fi. Would not pick it as a seminal decade film though, as the others below are.

60s - Easy Rider

70s - Texas Chainsaw Massacre, tough choice with Exorcist in there.

80s - Raiders of the Lost Arc, et al

90s - Jurassic Park, agreed, due to the use of CGI at such a level.

00s - Sorry, LOTR, I take the 1st Harry Potter and the series in this decade, but it's close.

While separating by decades is a useful and natural method, one of the problems with this is that seminal films and film trends are not decade bound. Some trends transcend decades and some are shorter periods within a decade. I would also say that Psycho is an important choice from the 1960s, but it really effected horror film styles much later on.