Of course, we are assuming that if you're reading this website, you don't have just a nice suit and black tie... which would give you all sorts of options: Jules and Vince from Pulp Fiction, any of the Reservoir Dogs, a Blues Brother, and so forth.
Now for your sartorial pleasure, our suggestions:
The Warriors - one of the Orphans
What you need: A grubby green t-shirt, blue jeans
Bonus if you get: A wonky eye
Last Action Hero - Jack Slater
What you need: A red t-shirt, blue jeans, big belt buckle, brown jacket, cowboy boots
Bonus if you get: Steroids, Austin O'Brien (trust us, he's not doing anything he can't cancel for your Halloween party)
They Live - Nada
What you need: A flannel shirt, blue jeans, sunglasses, a mullet (or a mullet wig if you lack dignity)
Bonus if you get: To uncover an actual conspiracy
Happy Gilmore - Happy Gilmore
What you need: A red t-shirt, a flannel shirt, a golf club or hockey stick
Bonus if you get: Umm... an alligator would be pretty cool! Right, guys?
The Big Lebowski - Jeff "The Dude" Lebowski
What you need: A grey t-shirt, a grey hoodie, board shorts, flip flops, a white russian (or you could go the more played-out bathrobe route)
Bonus if you get: If you can pull off the hair and beard for this costume, you get your bonus when, after Halloween, you shave and get a job, sir!
And remember fellas, if you're feeling really lazy, you can just go with a white tank top. You can claim all sorts of characters that way!