Although the TARDIS is technically a time machine, for most episodes the Doctor could rock up in a bus and it wouldn’t change anything.
The most obvious answer for me is Blink, with the conversation on the DVD Easter egg. Very cool use of time travel, and seeing it come together is brilliant.
I’ll also add Waters of Mars. It’s not really got any time travel in, but I feel like the foreknowledge which shapes the plot just about counts.
What are your favourites and why?
“A Christmas Carol” springs to mind as one of my favorites with in-episode time travel. What’s more Doctor than trying to save the day by retroactively making the villain your friend?