Hmm.. whatever edition of Basic D&D had a red cover. I still have the book, but alas, the cover is long since fallen off and disappeared. At least, I think that's the oldest. I have the original Traveller box somewhere around here, too.
Somewhere around I have a copy of the original Chainmail rules, although I've picked that up since for the heck of it. The oldest games I have that I've actually bought and used new are the old black book Traveller (not first printing) and a first printing of the old staple-bound Champions. Don't know the date on that one, but if can't be later than 1982 or so.