I've been wanting to do a list like this since I did my top 10 books and manga I reviewed. I would like to note that I only allowed two episodes per series to make this list because I wanted the list to have some variety.
1. The Fades(1.04): This episode was just really interesting since it had the main character be pretty much dead for most of the episode but he was also kind of like a ghost so he could see everything that was going on this episode. I also really liked all the plot development with the fades that occurred in this episode and I believe that this is the first episode were we learn about who John is and I've always thought of him as a great character. I just overall really loved this episode but I feel that this episode and the next episode are pretty much tied.
2. The Fades(1.05): I love this episode because I felt like a lot of things happened in this episode and I liked that by the end of the episode pretty much everyone knows about what's going on. I also liked that there were quite a few character moments as well.
3. Vampire Diaries:1912(3.16): I loved this episode because it mostly focused on Damon and Stefan's relationship with each other in both the past and the present and it had little or nothing to do with Elena for once. I also just loved that Damon felt like Damon in this episode because I felt like for most of season 3 he was just trying to be who Elena wanted him to be but in this episode he didn't care about her opinion at all. I also found the twist at the end of the episode with Alaric to be very surprising and interesting.
4. Teen Wolf: Frenemy(2.06): I just pretty much loved this episode from start to finish and I love how the plot pretty just kept moving during this episode. I also found that there were quite a few character and relationship moments I loved as well but mostly I can just remember loving the episode as a whole.
5. Teen Wolf: Fury(2.10): I loved this episode because took place in a very small amount of time and revealed a lot about the Kanima and also why Matt wanted the swim team dead. I also thought it was kind of one of those episodes that pretty much set up the last two episodes of the season.
6. Revenge:Duress(1.11): I was just really entertained by Tyler completely loosing it and just generally acting insane. I just remember completely loving this episode but I'll admit that it's been awhile since I've seen it.
7. Vampire Diaries: Before Sunset(3.21):The second to the last episode of the season for the second season in a row is one of the best episodes in my opinion. I love this episode because it sets itself up for great season finale(although the season finale actually wasn't all that good) and it basically the plot moved forward a lot in this episode. I also really loved the scene where Klaus tries to get invited into the Gilbert house by doing things such as ripping apart of a fence and throwing them into the house or coming up to the house with a newspaper that's on fire, this scene is ridiculous but it's also my favorite scene of the season.
8. Revenge: Destiny(2.01): The season two premiere of Revenge was great with the plot still being interesting and there were plenty of scenes between Emily and Nolan which is my favorite relationship on the show to watch. I also loved that the show was able to make me believe that Victoria was dead for half an episode and I loved almost every plot this episode.
9. Grimm: Woman in Black(1.22): This was just I really good season finale and I love that it had like two or three cliffhangers because it just made that much more excited for the season 2 premiere which was great by the way.
10. Merlin: The Darkest Hour(4.01-.02): I thought it was a good story to set up the season although I find it a bit strange that show decided to skip over a whole year but I guess the show just likes to do that. I liked the overall plot of this episode and I especially loved all the interactions between Merlin and Arthur but I almost always love all their scenes together. I also loved that Merlin was able to talk to Lancelot about his magic throughout this episode because I just liked that Merlin had a friend that he could be honest with. I also liked that in this episode there were parts that showed that Morgana still had emotions and wasn't just some villain which alot of other episodes seem to have a problem portraying this.
There's a lot of other episodes I loved and reviewed and I'm not exactly sure of the order of this list so don't be surprised if my top episodes of 2012 list looks a lot different than this list.