I think the big one is Halo. I doubt I will ever fanboy for a series as much as I did (and I guess continue to) fanboy for Halo. Religiously played all the games, went over the cutscenes in extensive detail, checked out as many obscure battles and information I could find on as much as I could. I was a member of Halo Fanon for a good few years until I started running out of ideas to defile with my putrid writing skills, and I think their Canon Policy is what got me so invested. I've read about seven of the books, a graphic novel or two, and Halo: Contact Harvest about three times.
Then an issue arose. Let's just say that things were looking troubling from 2011 onwards and then on the 6th November 2012 I understood what Star Wars fans felt about the prequels. You know that horrific, childish intro to MovieBob's Amazing Spider-Man 2 review? I genuinely felt that shit which was probably why I wasn't harsh on him for it. But hey, at least when I revisited the games in the MCC last year I could enjoy speaking the entire script to near enough every cutscene (especially in Halo 2 and 3) and I came at... the closest I'll ever get to peace with that thing they shat on me.
There are other things I fangirl for, the closest in terms of "obsession" I suppose would be over analysing details, statistics and background information on the career and tours of Lady Gaga, but nothing I'd really stand on the hill and die for.