For me, it would be My Little Pony: Friendship is Magic. When all of the hype and counter-hype was going on, I watched from the neutral sidelines, eating popcorn, not judging, but not on either side. When I saw fanmade videos, I appreciated the work put into them, but still didn't. Finally, last November, I was bored one night, and just randomly got the urge to check it out, just to see what all the fuss was about. "I'll watch one episode," I thought. And then, "Well, it's a two parter, so I have to see both parts." That night, I watched 5 episodes. I am currently partway through Season 2, and have stopped watching it more out of a lack of time and too many current things to keep track of than actually losing interest.
Also, Game of Thrones. My sister/brother-in-law insisted I read the books. I didn't take at first, but they showed me the first episode of the show. I have finished reading all 5 books so far, and am eagerly awaiting the sixth.