Easily Dungeons and Dragons, mostly AD&1e, but I also had the Basic and Expert boxed sets and some B/X modules. For AD&D1e I had all the hardcovers, a bunch of modules, Dragon issues, issues of some other magazines that covered D&D ... just a ton of stuff.
But this was the 80's, the era of the Satanic Panic, and some relatives who literally lived on a bible camp came to our home and told some bold-faced lies to my parents about D&D. I tried to argue against it, saying that I had read the books cover to cover and what they described just wasn't in there. My parents chose to believe the adult relatives over me, and threw out my collection.
I was able to secretly fish the books out of the trash (we were moving and it was all dry trash in there) and hide them for the next couple years, adding to the collection. I would secretly play at school during lunch. In the end, though, it was discovered and I had to get rid of it once and for all.
More recently, I've re-acquired some of the books, mainly the core hard-covers (PHB, DMG, MM and DDG), a couple modules (ToEE and GDQ1-7), and the Basic boxed set (which included a set of original dice!). I've looked longingly at eBay listings for some of my favourite Dragon issues, but it's hard to justify the expense. And there are things I had to get rid of that I simply cannot remember the name of (like a non-Dragon magazine with a cool solo adventure).
