For me it's Pitfall on the 2600. I think it's charming, replayable, and it can keep me entertained far longer than most modern AAA slop. Good example of a game that's merely a game and nothing more.
lemmings is alright, I think I played it in my browser when I was, like, a child, or something
otherwise the oldest I've played enough to have an opinion on are from the early 2000s not counting nintendo shit