I really liked the episode of House where Amber dies:
That's the first time I ever heard that song, Iron & Wine's Passing Afternoon... I love it now.
I also loved the Sherlock episode with Irene Adler, "A Scandal in Belgravia." Maybe not as profound, but entertaining.
I also loved the first season episode of the Sopranos where Christopher Moltisanti asks, "Where's my arc?" (The Legend of Tennessee Moltisanti) I quote that all the time...
What's the best episode(s) of tv you've ever watched?