1.3, in my opinion.
Well, 1.03c to be precise.
Then 1.4 (or 1.04) gets to be a bit harder. You are pretty much forced to upgrade your infrastructure before you can industrialize, especially playing a minor nation. But the hardest nerf was no longer being able to concentrate on researching culture to gain prestige and then trading the culture for military techs. Drove a very popular strategy to its grave.
Then Revolutions made it all easier again.