The Dalek Invasion of Earth

This is one of the big ones.  A sweeping, incredibly ambitious epic centering around the first return of a previous villain - and the villains who put the show on the map.  It takes everything the show learned across the first season and change and then shoots for the moon.  And it comes close enough to hitting it that it deserves to have the word "Classic" thrown at it.  There are a lot of little things wrong with The Dalek Invasion of Earth, but it gets all the big things right - and one above all that makes this the first episode of Doctor Who that truly feels fully like Who.

Planet of Giants

Susan: Can you get us back to normal size?
Doctor: Well, of course!  Yes, I'll try.  There's always a chance.

The Reign of Terror

Barbara: What have you done?
Ian: Oh, nothing much.

The Reign of Terror, the final story of Doctor Who's first season, introduces writer Dennis Spooner, who became the script editor for the majority of the second season, and would end up writing The Romans, The Meddling Monk, and the back half of The Daleks Master Plan, as well as contributing to the first episode of Power of the Daleks, becoming one of the best and most important figures in Doctor Who's history.  Which you never would have guessed based on his first Who script.

Irrelevant Side Trip: Men In Black III

Men In Black III is surprisingly good - fast, funny, clever, and featuring an amazing performance by Josh Brolin as the young Tommy Lee Jones.  Heck, Brolin does a better Tommy Lee Jones than Jones himself has done in years.  Everything the second film did wrong is fixed, and while it's not as good as the original, it's a highly enjoyable flick.  Of course, if, for some reason, you haven't seen The Avengers yet, that's the alien invasion movie to see.  In fact, if you've already seen it, you can safely see it a second time before MIBIII is particularly necessary.  Still, it's good stuff.

The reason I bring it up, though, is that the plot is involves putting a high-tech futuristic device on Apollo 11 to save the world from an alien invasion.  Which Doctor Who, of course, already did last year. Better.  And included an awesome cameo from Nixon. (though this film has a fun riff on another '60s icon)

So, yeah, Doctor Who reaffirms itself as the greatest thing in the world.

The Sensorites

Doctor: I think it was an exercise in fear, in power... They can control, they can frighten, yet they don't attempt to kill you.  Furthermore, they feed you and keep you alive.  Oh, this is most extraordinary!

During the course of The Sensorites, Susan comes incredibly, painfully close to working as a companion.  There are problems with how she's done here, but she just about fits, to the point that you can actually see how she could have worked.

Of course, the next serial goes and melts her wings, but we'll pretend that doesn't happen for a moment.

The Aztecs

Autloc: It is our law that all who attain their 52nd year shall pleasurably pass the remainder of their lives free from responsibility and care.
Doctor: Poor old souls, they must be bored to tears doing nothing.

After Marco Polo managed to weave a generally excellent yarn that seemed completely irrelevant to the show and Keys of Marinus managed to do exactly what Doctor Who should be doing and somehow failed to be good, it's strange to see Polo's John Lucarotti return to a pure historical and actually make a better Doctor Who story than Keys.  The Aztecs not only has all the rich characterizations and dialogue of Marco Polo compressed into a much more tightly-plotted and exciting adventure, but even without the crazed imagination, it manages to be a better science fiction story - and a vastly more compelling addition to Doctor Who itself - than Keys.

The Keys of Marinus

Writer Terry Nation cleverly structures Keys as six individual mini-adventures, each with its own unique setting and challenges.  It's a saavy move that proves he really does understand Doctor Who - particularly its ability to tell absolutely any story in any setting..  And then he goes an totally misses the point he just got by not making the TARDIS crew travel by TARDIS.

But it's probably best to just take each episode individually and judge them on their own merits: