Best single episode of Doctor Who?

The Destruction of Time (DMP, Episode 12). Edge of the seat stuff from start to finish. Only the fourth part of Remembrance of the Daleks matches it in terms of suspense and drama.

Caves episode 3.
Logopolis episode 1 ("so he did escape from Traken", also worth it for Tom's delivery of the line "standing on their heads is an expression" where it looked he wanted to throttle Adric)

Part 3 of Two Doctors is another one. I just love seeing Pat and Colin piss about

The Talons of Weng-Chiang Episode 1.

A few nominations: War Games episode 2, Earthshock episode 1, Robots of Death episode 2 (with the "she's GOING!" cliffhanger), Caves of Androzani episode 3, Warriors Gate episode 1 for the sheer weirdness factor - the seemingly normal Captain's crew just exacerbates this somehow - the episode of Inferno when Pertwee arrives on the alternate Earth, Revelation Of The Daleks 2nd half, and a nod towards Fenric episode 1 - creepy stuff.