Hard to say. In a technical sense it's probably something I watched a lot as a kid just because it was around and I liked it, like Cabin Boy or something.
But having grown up a bit and thinking only of "serious" viewings where I was paying a fair amount of attention, there's probably a four-way tie (or nearly so) between Up, The Lord of the Rings: The Fellowship of the Ring, The Dark Knight, The Departed, each of which I've probably seen about 7-8 times, give or take. The Shawshank Redemption is probably one or two viewings back.
But having grown up a bit and thinking only of "serious" viewings where I was paying a fair amount of attention, there's probably a four-way tie (or nearly so) between Up, The Lord of the Rings: The Fellowship of the Ring, The Dark Knight, The Departed, each of which I've probably seen about 7-8 times, give or take. The Shawshank Redemption is probably one or two viewings back.