I collect mainly big box PC games. In spite of being American, and being a le heckin child of the 90s, I never owned a console until I was an adult, and always played shit on PC. Started on the family computer then gradually was able to get into hardware acceleration. So, that's what most of my collection is, old PC games. But big box PC games are awesome, and if you have any passing interest in the golden era of of computer gaming (roughly 1989-early 2000s) Get some big boxes. A lot of them are still cheap.
Ones that I have that are expensive now are:
- Quake (no idea why it's expensive but it is if you look at ebay prices)
- All four of the mainline Mechwarrior games including the expansion packs
- LOOM, Monkey Island, Day of the Tentacle, Full Throttle, and other LucasArts adventure games.
- The Neverhood
- Blood and its expansions which are very rare.
- American Mcgee's Alice
- various Ultima games since Origin went all out with presentation
- The weird but cool Eidos trapezoidal boxes, like the Thief games and Tomb Raider games
- System Shock 2




I mostly look for expansion packs now since I have most of the games I want, and expansions are more rare and hard to find.