Primer is an excellent movie. It is clever, complex and very interesting and you need to be stone cold sober to follow it but it is highly rewarding , even after repeated viewings. Techy people who went through the dotcom bubble will really relate to it.
The fact it was done on a teeny weeny budget with unknown actors is highly impressive.
If you watch it, grasp all the time lines and then your brains melt don't blame me, there are worse ways to become a vegetable.
(After watching it and still not following the various threads there is good plotline diagram on the web but you need a huge printer to do it justice.)
And an old list from 1999 was:
Blade Runner - gets my vote for believable setting
Appocolype Now - believable setting I'd rather not be in
A Clockwork Orange - nice setting and people

Romper Stomper - best closing credits of a movie
2001: A Space Oddyssy - unbelievable effects for it's time
Christiane F - Excellent music , realistic atmosphere
Animal House - funny movie, funny people
The Graduate - interesting plot
The Wall - too deep to understand at one sitting, or two, or three..
Spawn (Directors cut)
Dark City
The Matrix
I'd add a lot more Rubber Reality movies these days, being a fave genre.
http://www.imdb.com/list/_-AB0GMQ6mA/