MLP. I've been a closet brony since 2011, and while my interest in the show has waned, I still read the occasional fanfic, participate in some MLP-related communities (this one and the MLP parts of Reddit,) write excessively long bits of lore, and make vectors for my OCs, so I've been going on five years now.
By contrast, I was in the Sonic fandom for only about three years (2008-2011), and I'm not anything close to being a "member" of other fanbases.