Heroes of Might and Magic III
I couldn’t begin to count how many hours I played this back when it was new. Heroes IV was fun too, but this is the one that I always remember from the series. Good for both solo play and two-player campaigns, the key is really logistics: what resources do you go after first, which troops do you build up, where do you send them, and so on. There’s strategy to the actual battles too, but it mainly depends on your choices building and exploring.