Poland, 1441. I'm shocked at how easy it is. Yet surprisingly fun to juggle so many BB wars at once (not noticeable at here since i just beat all my neighbors into submission and am waiting for truces to expire to beat the crap out of them again) Green is vassals and PUs.
Started with the mission that gives me a CB on Mazovia. DoW'd them, Hungary answered the CtA. Vassalized Mazovia (and later diplo-annexed it) followed by the swift annexation of over half of Hungary's territory. Im shocked at how loyal Lithuania is in DW, haha. After that, got the mission to subjugate Teutons. Took a quarter of their territory in one war, and vassalized their allies (I believe Pommerania and Brandenburg, not sure). In the next war, took a province from Bohemia who was their ally, and Danzig from the Teutons. Then, got a CtA to fight Hungary, answered it, vassalized them (because of revolts they were down to three provinces) and Croatia, who jumped in too. Got a PU with Norway, who's proven useful so many times. Just recently have been beating up Austria, if you were wondering those two countries inside their territory are Tirol and Styria. I vassalized Aquilea as well in the last war. Now I'm pretty much unstoppable, I should be able to conquer all of the HRE by 1500.
I'm at permanent -3 stability from a combination of Peasant's War, BB events, and endless streams of random events that make me lose stability (no comet sighted yet though, haha) but even then, my yearly income is ridiculous because of vassals. I have a high BB, but that's only allowing me to get into constant wars even with -3 stability. I was also blessed with a 6 shock, 5 fire, 3 maneuver general. And he's my leader, so his chances of dying are slimmer than a regular general (as far as I know). In five years, I'll be able to vassalize Austria, who's currently the HRE. Once the crown passes to either Bavaria or Bohemia, I'm sure they'll attack me too.(3/4/9 iirc)