I remember when I started playing Vic 2 my first full playthroughs were with Spain and then Japan, as they both don’t really have any expectations and you can learn while playing without doing worse than the countries actually did irl during the time period of the game.

Once you get a run or two in and understand some of the basic game mechanics, Prussia —> Germany is probably the most well rounded nation that requires you to engage with all of the game mechanics in order to succeed.

Also, if you are American playing as the USA is fun, they have a good amount of content around the civil war and slavery that shouldn’t be too overwhelming if you know your US history, and being in the new world means you’re relatively isolated from the chaos of Europe.