I'm going to be really miffed when people start coming into this thread and posting rifles that aren't assault rifles because they don't know the difference.
Having said that, I've always been a fan of the Tar 21.
Not really because of any game or movie, but because the gun just appeals to me on an aesthetic level.
I have a thing for bullpup rifle designs. It's not a pretty gun by any means, but it isn't supposed to be, it's not an elegant weapon, it's a weapon for the battlefield, it looks like it can take a lot of punishment and deal out even more. Sadly I haven't had a chance to shoot one.
On the other hand, if you'd asked me this a few years back I would have said the Steyr AUG, but having shot one I was incredibly disappointed with its performance (damn thing jammed on me 4 times in 20 rounds), so shooting the Tar 21 might not leave as favorable an impression on me as I would hope.
The truth is, I've shot my share of assault rifles, but I've never really enjoyed them all that much. I always prefer their civilian semi-automatic sport rifle counterparts.