Definitely Heroes of Might & Magic 1 for me, on our 486DX2-66.
The game ran borderline laggy, but my brother and I spent more than a year playing the campaigns over and over using different builds, lots of hot-seat games when we both wanted to play.
In fact, my first-ever website (more like 6 simple webpages and a free counter) was about HoMM1, and I remember dreaming I'd wake up one day to find 30,000 hits overnight :) I put up some random-generated maps, too, that people told me were super awesome (LOL).