Hands down, League of Legends dominated PAX East 2015. Even though Riot Games was absent from the show floor, their League of Legends "cospitality lounge" served as a place for all fans to gather, celebrate their wonderful costumes, and get their picture taken in front of some sweet artwork. The room was only open to cosplayers, but an entire gallery can be seen online. Pretty impressive stuff.
Aside from League of Legends though, day three of PAX East included quite a few other impressive outfits, including some awesome Halo Spartan suits and my personal favorite, Kingdom Hearts shoes! Sure, shoes aren't really cosplaying, but they were freaking awesome!
You know what else was awesome? Talking to all you find folks about your favorite game(s) at PAX East 2015.