Noble mission: to chart out which alternative versions of movies are the definitive ones to me. Means: just because you add crap doesn't mean you just made the film superior. Yes, the statement is obvious, but at least this list entertains me for now. The longer versions are usually interesting - yes, but the additions quite often fuck up the balance of the film. Not that you shouldn't view the long versions in these cases; just that you should view the theatrical version first.

The Abyss - both're crap
Aliens - theatrical, although many of the SE additions are great
Army of Darkness - longest cut, but endings are equally great in their own ways
Donnie Darko - theatrical, because the director's cut is essentially a SE
Blade Runner - the final cut, fucker
Leon - Version integrale, hands-down
Star Wars IV-VI - no need to answer
T2 - undecided