...that you never actually play? Whether it's a 'just collecting', 'haven't played in ages', 'can't find a group', whatever.
Me, it's Modiphius' Star Trek Adventures. After I posted the last thread about Trek games, I was able to get a good deal on most of the books; I think the only things I'm missing are the Player's/GM's Guides, the Klingon core book, and the Disco sourcebook; and I'm not too broken up about missing those. Even scored the tricorder collector's set!
Got to thinking about it because Modiphius announced STA 2E recently, supposedly back-compatible with all the 1E books, but they've already previewed a couple pretty significant mechanical changes and I'm wondering how the conversion is meant to work. Surely they'll get to that in later previews.