Harry Potter is a great series, but Rickman's character is too unlikeable, as I'm sure he was intended to be - blaming a kid for his father's sins when the kid never even knew his father. He's supposed to be smart, but then again, he's Slytherin.
Die Hard is a fantastic stand-alone movie, and Rickman's character is a fantastic and far more believable villain with better understood, albeit sociopathic motives.
But I'm still going with Galaxy Quest for the fantastic parody and humor.

Of course, as a Star Trek fan, how could one not?
R.I.P., Alan Rickman. Cancer sucks.