CELTICEMPIRE
Zulu Conqueror
As someone who has played most of the games in the series, I think it has to be the variety. There's a variety of Civs and a variety of strategies that keeps me coming back.
I believe the major advantage of the Civilization series compared to games such as EU is simply... freedom. You build everything by your own, from scratch. From hunting and gathering to the space race.
From that point of view, I wouldn't say Civilization is necessarily a historical simulation like Europa Universalis is. There is a creation/construction aspect in it which makes it maybe closer to games like city builders or transport tycoon games where you actually grow by yourself something complex out of nothing.
I can't talk for others, but at least in my case, that's what I find the most enjoyable.