I find that most people in my life have at least one "thing" they are collecting. It is often an insight into their personality and it is fun to hear people talk about something they are passionate about.
I collect ttrpgs! I love reading how mechanics work together and I've never found value in collecting things I don't use, so ttrpgs it is. Plus there are some really wild ones out there that are fun to look at. It started with 3.5/Pathfinder but eventually I branched out to a couple other games that I like to play. I spend the most effort on my Car Wars collection, think I'm going to try getting a complete set of ADQ on that front next.
My single prized book so far though is a copy of HōL's supplement buttery wholesomeness. Specifically the second printing of it with the cover that's written like BUTTery HOLsomeness which ended up being so hard to track down over like 5 years that I started to question if it even had a physical printing or if it was just in PDF form. Finally found the thing though last year
I personally love Paranoia. It is very much a comedy game but it can be played with a serious take with the right work. Plus it is just a fun read even if you don't play it.
Rifts is pretty baller conceptually but I don't have first hand experience playing it.
GURPS has a Car Wars crossover making it objectively one of the best games, can be played with any level of tech, and has buckets of supplements.
The Burning Wheel is more fantasy but the book is basically a philosophy of play which could be adapted to lots of things, and the same author wrote Burning Empires which is more scifi but I haven't acquired that one so can't give first hand info on it either.
Can't forget Car Wars. It is scifi though it is a mix of ttrpg and strategy/board. If you want to drive a car off a cliff while dropping mines on a helicopter as you go by and have your robot gunner leaning out the window shooting lasers at a tank it is the way to go.
Finally I'll pitch HōL because the game is just so unhinged. It is all hand written and scanned, it's edgy and crass, and is a great parody of roleplaying games. It's engaging and fun to read with great art but realistically quite difficult to functionally play because the rules are intentionally a mess. The Buttery Wholesomeness supplement added character creation rules which don't exist in the base game because it is a mess.
I'm rather proud of it. Resolution isn't great but here we go:
Main roleplaying shelf:
Board games and box sets:
I've also got all 3 books for Monsters of Murka, limited covers, signed by several of the people who worked on it. I think my copies of Thirsty Sword Lesbians and Mork Borg are on other shelves. Finally, an unopened box set for Boat Wars which I'll be opening sometime soon so I can pretend I lived in the 80s and just came home with a box from the lgs.