Yikes! This is hard - my list changes with my mood, but at the moment this would be my top 6, in no order:
The Goonies
The Labyrinth
It's a Wonderful Life
Clueless
LOTR (I'm counting the trilogy as one film!)
From Dusk Til Dawn
The way I see it, the more often you can watch a film without getting bored, then it must be a favourite. But then, there are films I don't particularly like watching, such as Casino (head in a vice! head in a vice!) but still love. So I've argued myself out of my original statement. Hmm. I don't know what defines favourite films then.