It's hard to pick my favorite episodes, because when I'm watching a marathon, my reaction is kind of like, "Wow, this is a good one," at every other episode. And the ones I've watched for potential props are not necessarily my favorite episodes overall. Right now, there are a few that come to mind - "A Game of Pool," "5 Characters in Search of an Exit," "The Obsolete Man" and "Night Call"(I think that's the name - it's the old woman who's been in several episodes who keeps getting bizarre phone calls at night). And then there are the ones that everyone loves, like "Time Enough at Last," "The Ventriloquist," "To Serve Man," etc.
Once in a while, I catch one that I haven't seen before, and it's a special treat, even if it's not one of the best episodes.