As I mentioned yesterday, this was incredibly difficult. Therefore, I will list my favorite episodes from Series 1-6 of the new Who. I have not seen enough of Classic Who to really name favorites.
Series 1 (Christopher Eccleston): As much as I enjoyed "Father's Day," I will have to go with the two-parter "The Empty Child/The Doctor Dances" for introducing us to Steven Moffat, Evil Genius and Creator of Nightmares. It also gave us a line to quote whenever we see someone with a gas mask: Are you my mummy? Oh, and this is also the first time we meet Captain Jack Harkness.
Series 2 (David Tennant): I am really torn between another pair of back-to-back episodes, but in this case they are two separate stories. I suppose I will have to go with another Moffat creation, "Girl in the Fireplace," because I covered "School Reunion" pretty thoroughly a couple days ago. The clockwork demons are some of the creepiest Who villains I have ever seen. It must be their empty eyes and jerky movements, straight out of the Uncanny Valley. I loved the scene at the end where the Doctor comes bursting in to save Reinette on horseback. Through a mirror. Oh, and I recognized the actress who played Reinette from the underrated, unfairly cancelled Moonlight.
Series 3: Another two-parter: "Human Nature/The Family of Blood." I loved seeing Human!Doctor. I thought it was cleverly plotted, and because they had twice as much time, I was really able to get a sense of the historical atmosphere. I know some fans are not too crazy about Martha, because of the whole unrequited love subplot that ran through the series, but I have nothing against her. I thought it was handled particularly well here. Oh, and this scene was done so that they would have something for Martha to fast-forward through while watching the Doctor's video. I find it absolutely hilarious.
Series 4: So many good episodes here! For a lighthearted favorite, you cannot beat "Partners in Crime," which I discussed when explaining why Donna is my favorite companion. I actually really liked "Turn Left." Although it is one of the darkest episodes of the whole show, I am a sucker for alternate realities (see "Father's Day" in s1). Jason and I were watching it together, and he pointed out that the problem with alternate continuities is that they are always solved by pushing the Magic Reset Button. Of course, if you make your alternate reality as horrible as the one shown in "Turn Left," the reset button comes as a relief.
Series 5 (Matt Smith): New series, new Doctor, new companions...my favorite episode is, by a wide margin, "Vincent and the Doctor." I have always been intrigued by Van Gogh, and I love the historical facts thrown in along with the invisible monster and time travelers. It reminds me of the original intent of Doctor Who, which was to be an educational program with fantasy/sci-fi elements thrown in. Kind of like a live-action version of The Magic School Bus, but British. And in 1963. Anyway, the characterization in this episode was magnificent. The ending nearly made me cry. Beautiful and tragic, like the painter they focused on.
Series 6: We are only halfway through s6, so my answer may change by the time we get to the season finale, but as it stands right now, my favorite is "The Doctor's Wife." You can read the review from a couple weeks ago for more info.
Tomorrow's topic: Favorite series/season. Hmm... will this be easier or harder?