Trivial Pursuit games, but only the traditional rule versions and official US sets. I once thought I had all of the major original sets, then I spent a good amount of time researching the entire collection. My personal database is more accurate than anything online. :nerd:
But I'm up to 19 complete sets, with 7 of them being very hard to find, and 3 being damn near impossible. I have 51 more sets to go, 5 being truly impossible to get(AFAIK, because I haven't even seen them listed in ebay's history in any condition(complete or incomplete) for the last 12 months).