Well the series isn't actually done yet, so I'm gonna say Buffy the Vampire Slayer, however we're gonna go with the finale of season 5 cause we all know it should've ended there except for Once More With Feeling. (Sorry all you Buffy/Spike shippers)
The Vampire Diaries! Never missed a single episode! Bawled like a big ole baby for the series finale. Because it’s a BIG cast reunion at the end. Cast members you haven’t even seen in years! But, I was very invested in the show