I discovered today that my most played game on Steam is Astronner with about 500 hours, and my second most played is Surviving Mars with about 500 hours.
Must be Civ IV - that whole series makes you lose track of time - start a game and that's the whole weekend gone; even now Civ IV with the Realism Invictus mod feels like it's got tonnes of gameplay still left in it for me.
That said, OpenTTD has been an enduring favourite of mine for the past few years and because of its flexibility it might surpass it in the long-term.