I just wanted to reiterate that Donnie Darko is an incredibly dumb movie. (I bring this up because the director's cut is playing at a local theater.) The director may be able to describe (e.g. on the commentary track) a bunch of things that are supposed to be going on in the story that make it less dumb and more interesting, but sadly, he did not put those things in the movie itself (at least not the original, and I'm not interested in spending money on the director's cut to find out if it's different) so instead you're left with a movie that (a) makes little sense, (b) has a protagonist who does essentially nothing, and (c) those first two flaws can only be explained by introducing elements which just aren't in (or implied by) the movie. In which case, hey, you can make up anything you like. Maybe the whole story was actually a really bad dream Superman had while napping in the Fortress of Solitude. Wait, that theory is surely wrong... I don't think Superman takes naps.
I have a similar problem with Memento (it seems likely the director intended a particular thing to be true, but there's no real reason to believe Teddy is finally telling the truth at the end of the movie). In this case, however, the movie is perfectly fine even with the ambiguity; its story isn't nonsensical, it's just got an unexplained backstory.
Somehow this results in Memento being at +4 and Donnie Darko being at -2 on my -4 to 4 scale. (It's probably not really -2, but it keeps getting pushed there when I read people theorizing about it despite problem (c) above.)