1. Faith's character arc(seasons 3/4 of Buffy the Vampire Slayer and season 1 of Angel): I really loved watching Faith's character breakdown and develop overtime it's pretty much my favorite arc of all time. I really loved watching how Faith dealt with accidentally killing that guy in pretty much the worse way possible by running away from feelings of guilt and even joining the evil side. I thought her whole arc was brilliant but I especially loved seeing her become more self destructive and reckless in the Angel episode Five by Five and I loved that in the next episode she was finally able to take responsibility for her action. I just loved everything about what the show choose to do with Faith's character with this arc because it's absolutely brilliant.
2. Daenerys season 1 character arc(Game of Thornes): I loved watching Daenerys go from a scared and fragile girl to a confident and powerful warrior queen over the course of one season. I loved everything about this arc and I loved watching her character grow and I think this arc is just one of the best to watch.
3. Angelus arc(Buffy the Vampire Slayer): I really loved this plot because I was so emotional invested in this part of the show and this was really a turning point for the show in general with the whole show becoming much darker during this arc. I also really just loved Angelus as a villain because he saw killing and torture as an art form which made him all the more scary.
4. The cracks in the universe arc(Doctor Who series 5): I just really loved the way that this plot unfolded I like that there were little hints about what the cracks in the universe were and what they were doing all through out the series. I also loved how everything ended with the whole universe almost collapsing and I loved the last two episodes of this series the most because they were such great episodes.
5. Eye Opening Arc(Higurashi No Naku Koro Ni): I just really loved watching Shion breakdown and go insane during this arc, in a show were everyone goes insane and kills people Shion's some how stood out to me. I feel like I love this arc so much because this is weirdly the arc that made me love and sympathise with Shion. I loved seeing who Shion was and how she loved Satoshi and how that was one of the main reasons for her mental breakdown. I just loved this whole arc and I also loved that this is the first answer arc of the series.