(Note: I missed putting this one up yesterday because I was away from the internet. So, you all get TWO "30DoNW" posts! Lucky Everyone!)
I have to choose two for this - "Partners in Crime" and "The Eleventh Hour." If you end up watching these right after the preceding seasons and specials, both of these come off such emotionally wrenching stories and it's a good time to have some lighthearted fun and remember that, yes, Doctor Who can actually be an uplifting show.
In "Partners in Crime," Donna gets reintroduced which is probably the best possible thing for the Doctor at this point in his story - he's feeling guilty about how things went down with Martha and that little thing with Astrid (still wondering what was going on with that one). Who better to make things okay again than the enthusiastic, determined and hilarious Donna Noble?
"The Eleventh Hour" has the Doctor newly regenerated and everyone (including the audience) has been through the emotional meat grinder from "The Waters of Mars" to "The End of Time." It's about time we got back to the fun of saving the universe in twenty minutes or less (or it's free). Plus, we FINALLY get to have a happy ending (hooray!) and it's the beginning of a new season of "Doctor Who." This can never be a bad thing.