I'm on my third run-through of Star Trek: The Next Generation on DVD. Most of the episodes I had already seen on videotape several times before I got the DVDs. I seem to never get tired of this show. I'm watching two or three episodes a week so it will take me quite a while to get through it all again. I'm watching other new stuff as well. In fact I watch quite a bit more new stuff than I watch old stuff in general. Most series, even the ones I own on DVD, I only end up watching once. I buy DVDs mainly for convenience rather than rewatchability.
Ah, forgot about the Simpsons. I haven't done a run-through recently, but I've probably seen every episode from the 2nd to 9th season at least 4 times each.