Pretty much anything related to One Piece - manga, artwork, figures, books, soundtracks, etc. - all of that is probably my biggest collection. Been reading/watching for 20+ years at this point, story still hasn’t concluded!
I’ve also got a pretty massive Vinyl collection - a little bit of everything but mostly metal.
I guess I have a sizable Lego collection too, spend winters building the larger sets with the kiddo.
In the past I was actively collecting comics, movies, musical instruments, and Nintendo products. I’ve pretty much stopped on those collections as of now.
I find myself trying to ‘collect’ and I miss one crucial item or it becomes out of reach and then I abandon the collection. Still all sitting in the storage room though! Going to be a fun estate sale when I bite it.