Series 1 - The Empty Child/The Doctor Dances:
This is my favorite first series episode for no shortage of reasons such as it being the first appearance of Captain Jack Harkness and the first episode written by Steven Moffat, not to mention the incredibly scary little boy with a gas mask.
Series 2 - School Reunion:
A tough choice between this, The Girl in the Fireplace and The Satan Pit but School Reunion wins as it introduced me to Sarah Jane Smith and I loved how it all takes place in a school full of monsters.
Series 3 - Gridlock
I know many go with Blink as their favorite episode but I really enjoyed the plot of Gridlock and seeing New Earth's bad side was great, also I got to see David Tennant with a bunch of kittens which made my week.
Series 4 - Unicorn And The Wasp
The Doctor, Donna Noble and Agatha Christie side by side with a jewel thief and giant wasp - it was brilliant with a great twist to Agatha Christie's disappearance and it was great going back to the 1920's especially with all of Donna's humor.
Series 5 - The Pandorica Opens/The Big Bang
This was brilliant - Firstly we got Rory back then there was the doctor's amazing speech, also, it had tons of Daleks, Cybermen, Sontarans and many other terrible Doctor Who monsters and best of all it was was full of the timey wimey twists and turns that we love.
Series 6 - Lets Kill Hitler
So far this is my favorite episode of Doctor Who - I loved going back to Berlin in The Second World War and just about the whole episode was filled with memorable moments, flirting and best of all River Song who as well as the other Ponds we learned tons more about.
Series 7 - Dinosaurs On A Spaceship
The name says it all dinosaurs on a spaceship was an entire episode on a spaceship with queen Nefertiti, Rory's father: for the Sherlock fans, Rupert Graves and all the while there are robots and dinosaurs running around.
Series 8 - The Caretaker
For a long time, I would have said that Flatline was my favorite but recently I re-watched the caretaker and changed my mind it was interesting and funny while still having the 12th doctor being grumpy, the conflict between Danny and the Doctor was heartbreaking and entertaining from start to finish.
Series 9 - The Magician's Apprentice/ The Witch's Familiar
Series 9 was definitely not my favorite and I didn't really love any of the episodes but in this episode I did like Missy and Clara spending time together and I thought the relationship between them was perfect also I loved the speech the doctor gave to Davros.