I must admit, New Who leaves a lot to be desired in terms of eras. Hey, I'm not complaining because I love the show, but it is true (in my opinion).
So, two picks for this one:
First Doctor - The Romans story arc
I enjoyed this story - the First Doctor makes me giggle (good thing nothing has changed on that note eh?). I liked watching the companions and how they planted themselves in this era - full on with the costumes (even the Doctor!).
Ninth Doctor - Series 1 - The Unquiet Dead
It was the first episode of the new series to go back in the past. I liked watching Rose's awe at being able to do so and the Doctor's glee at showing time to someone else. Rose looked gorgeous in the ONE TIME she was able to dress up *sigh* Charles Dickens was a major bonus (loved the end where he asks for how long his work lives on for and the Doctor simply and happily says, "Forever." Yay for literature!). I wish they could have gone in the past some more. BTW, when I mean Rose got to "dress up" I mean in something classy-ish. Proceed.