Gotta go with District 9. It's pretty much got everything I need, from intelligent themes down to badass alien weapons and power armor. After that, Tremors is definitely my favourite movie franchise; it's got both nostalgia and an uncanny ability to still hold up to my more evolved tastes today, and Burt Gummer has to be the single greatest character I've encountered in any movie. Plus, unlike most monster movie franchises, it never got stagnant; every movie added something new or had a generally unique feel to it. Hell, the fourth one was even a straight-up western. With giant man-eating worms.