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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.
 
Then Revolutions made it all easier again.

Due to capitalist pops being able to build factories and railroads, depending on your party in power, I should note. The culture research nerf is still in place.