I don't know why someone hasn't said this already. Maybe it's just embarassing. And posters on this thread want to be too cool for school. Mentioning "Blink" is like saying you like sunshine and rainbows.
But "Blink" is THE episode that most Who believers would use to introduce someone to the Doctor. It's a nice bit of time travel stuff that challenges the brain when you first see it--so you have to see it again. But, as with many stories, it's not a lot about the Tardis or the Doctor or even his companion.