My big first fandom was the old anime The Slayers. It's not even a particularly good anime, and it hasn't aged terribly well, but it was my first "real" anime that got me hooked on anime and manga in the first place, and I dove reeeeaaaal deep into the fandom madness.
I wrote like a million fanfics on fanfiction.net (including more than my fair share of self-insert Mary Sue stories), I drew a ton of fanart and posted it all over deviantArt (shhhhh I still draw fanart for it sometimes), I drew a fancomic for a few months that never really went anywhere, and I built a website to catalogue all my favorite spells and characters from the show. I even met my current best friend on fanfiction.net and we would do Slayers role playing together in AOL Instant Messenger chat rooms.
Good times, y'all, good times. But also totally glad those times are well in my past!