There are many great movies out there and there are many, many awful movies out there. However, it seems that, in terms of ability to be watched, over and over, without becoming tired of it, a movie has to have some other quality to it that usually puts it closer to the awful end of the spectrum than the great end of the spectrum.
One of the earliest movies I recall attaining that status of infinitely repeatably-watchable was The Princess Bride. It was just one of those movies you could pop in the VCR (or stumble upon while flipping through the TV channels) and enjoyable watch it, no matter how many times you'd seen it before or at what part of the movie you started watching it. It's a pretty good movie, but it would probably never be considered a "great movie" (no one's ever gonna lump it in the same category as The Godfather).
The next movie I found that fell into this "infinitely repeatably-watchable" category is the movie The Fifth Element. It most definitely is not a great movie. Its plot and story are absurd. Its characters are barely two-dimensional, let alone having a quality that approaches "depth" - which, given the cast, is probably to be expected. The special effects are merely 'ok'. In sum, there's little about the movie that makes it "stand out". Overall, the movie is almost a pure style-piece. However, at the end of the day, it's the kind of movie that makes no pretensions about what it is, nor does it meander from trying to be one kind of movie or another. It's almost fascinating in just how "empty" it is. I don't know what it is that does it, but it's the kind of movie I can sit down and watch, over and over, and I can do so, no matter my mood or whether I've watched from the beginning or some mid-point.