S4's 'No Exit' - recommended
I highly recommend this episode. It works well as a stand-alone, for those of you dabbling in but not committed to the series, as you can't really place it in time, and it doesn't contain any significant spoilers. (A few things may just puzzle you, depending on how much you've seen.)
A thought-provoking ability is involved, and pretty much just the main characters. It has a 7.8 here, for what that's worth to you.