It's hard for me to say "Greatest Ever" but ...
I might have to throw out "The Fugitive" as a contender. It's one of those movies where I know it's a great movie, but I also know it's a serious roller-coaster, so I'm never quite in the right mood.
That is, until someone puts it on. I just watch ten seconds of that thing (and it doesn't matter how far into the movie it is) and there's no way I'm tearing myself away until Harrison Ford and Tommy Lee Jones are together in the car, pulling away from the hotel.
It's the movie I'm never in the mood for, but never more than ten seconds away from getting there.
Perfect score in my book.