I've played 1, 2, call to power (not sid miers), 3, missed 4 and 5, and have been playing 6 lately. 2 and 3 were my favorites so far. 6 just has WAY too much going on for my liking with all the culture and religion warfare aspects. So far, my MO has stayed the same- build as many cities as you can early on and then start concentrating on economic and military techs. Crank out a bunch of units, overwhelm the enemy and then just keep upgrading those units. I've had game plays on lower levels where I don't ever lose a single unit. You just march across the world with the same 15-20 units and crush the enemy. Once you get the jet bombers in 6, it's over for your rivals.