For me it was Civ 4's modability that made it the best for me. Because when I got tired of playing Civ 4's normal game, I could install the Fall From Heaven mod and play a completely different game. Wizards, golems, angels, demons, spells, wild animals instead of barbarians (which could be tamed and turned into your own units if you had units with the right promotions)... it made for a completely different gameplay experience.

If I hadn't quit computer games cold turkey (when I realized I was showing all the signs of addiction) over a decade ago, I would still have Civ IV installed and still be playing it today. It just didn't get old, because of how varied the game could become.