Even though I consume a lot of stuff that has a fandom surrounding it (mainly anime/manga and that sort of stuff) I rarely get involved with other fans and fandoms due to my shy nature and few other factors that tend to push me away from them, but I do engage in some fandoms much more than others:
1. Fate-series/Nasuverse/TYPE-MOON - Probably the fandom I'm most engaged to.
2. Loads of different anime-idol and music franchises - Love Live!, Idolm@ster, BanG Dream!, D4DJ, Vocaloid (mainly in the form of games), K-On!, PriPara, Aikatsu, Kiratto Pri-Chan etc.
3. Magical girl/Mahou-shoujo - Sailor Moon, PreCure, Revolutionary Girl Utena, Princess Tutu, Cardcaptor Sakura, Kageki Shoujo Revue Starlight etc.
Been starting to fall down a mecha-rabbit hole as well so we'll see how that will turn out.
I have some game fandoms that I am somewhat actively engaged to: Persona/SMT, Danganronpa, Yakuza-series, Handful of Nintendo IPs are probably the biggest ones. Also a handful of different gacha-game-based fandoms.
When I was younger I was also active in the usual suspects like Harry Potter, Star Wars, LOTR, Gravity Falls, Steven Universe and GoT to name a few, but not really anymore.