My favorite Civ was Civ IV, but Civ V is a very close second. But once the DLC's were put in, it made it feel more like IV but with extra toys and pretty UI.
Civ IV was a true pioneer in the series with all of the new features and improvements from the previous versions, and it was just beautiful and so much fun to play because it was extremely challenging, along with the balancing act you always had perform to keep your production high and people happy. The espionage was so awesome, too! Also, being able to convert enemy cities with your Culture was an awesome roundabout way to completely wipe out your enemies without going to war. Not only that but it allowed you to avoid angering other Civs by not being a warmonger, all the while the person you're taking over ends up getting the hostilities from other Civs because they end up being the aggressor towards you, and all you have to do is defend yourself.
It also had great modding capabilities and really opened that up going forward, with heavy emphasis on variety, openness, and unique gameplay strategies.
I've probably put about 300 hours into each one easily.