I tend to geek out over things that: 1) have relatable, interesting, complex characters. 2) make a good story, especially if they're complex or have a good twist. 3) have a good concept to them. 4) are my brand of fun.
Anime: Oofuri and One Piece
Books: the Thief series by Megan Whalen Turner
Shows: Sherlock
Cartoons: Avatar: the Last Airbender, Gravity Falls, and Steven Universe
Other: Welcome to Nightvale, my own writing/OCs
When I get to a certain point of geeking out, I end up buying an obsessive amount of merch.
Spoiler
It's hard for me to find anything Oofuri, but I do have a baseball, bat, and mitt keychain, as well as an actual bat and mitt.
I have a ton of One Piece merch; mostly keychains and straps, but also prints, toys, and other little things.
I collect my favorite books, including the entire series of the Thief.
I have Sherlock cookie cutters, DVDs, soundtracks, a shirt, the casebook, and some magnets. Most of these are gifts from my friends.
I don't have much Avatar stuff, but I did buy my brother the entire series for Christmas.
I have a plush of Waddles, the third journal, and a keychain.
I have no SU stuff! DX I would like a shirt, eventually.
I also have two large Night Vale and Desert Bluffs posters.