For me the most classic one is Civ III by a mile. 4 was way too modern/ flashy for me and 2 too old school. But maybe I was just born at the right time for 3.
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.
Around 30 to 35, don't remember which year it was exactly.
And the thing that was largest in making me go, "Yeah, this isn't good for me, I need to quit" was that it was consuming my thoughts all the time. When I wasn't in front of the computer gaming, I was thinking about the game and planning the strategy for my next move. (I usually played turn-based games rather than action games). Which is fine in small doses, but it was taking over my mind when I was at church wanting to focus on worshiping God, when I was at work (and distracting me from getting work done), when I was trying to read...
Basically, I realized that it was an unhealthy focus for me, and taking over way too much of my attention that I wanted to be able to spend on a much wider variety of things. So I quit. First year was the hardest, second and third years were hard too, but by now I've gotten used to reaching for a book to read rather than a game. And the book, I can put down anytime I need to, without feeling that empty-ish feeling that says "Awww, I want to get back into the game..." That letdown when I exited the game was another clue, BTW: it matched how I'd heard drug addicts (specifically, former addicts who had kicked their habit) describe the feeling of coming down off a high. I've never used drugs myself so I can't compare it directly, but it was similar enough to the descriptions I'd heard from them that I said "okay, that's probably not a good sign either."
