It's hard to just pick one. Ignoring all genres Army of Darkness is my absolute favorite of all time. But the thing is there are certain movies I can watch no matter what, over and over. I like to think those are my favorites of all time..such as:
Star Wars 4,5,6,7
The Thing
Return Of The Living Dead
Robocop
Conan
Back To The Future 1/2
Evil Dead 2
Army of Darkness
Grandmas Boy
Dumb and Dumber
40 Year Old Virgin
Tombstone
Platoon
Good, The Bad, The Ugly
Pulp Fiction
Reservoir Dogs
Patriot
Last Of The Mohicans
But Army of Darkness..it has had the biggest impact on my life as weird as that sounds. It had zombies, Bruce Campbell, Medieval Knights, good sound track, amazing ending, and the one liners from Ash..man I basically quote that movie on a weekly basis..Groovy.