Just a question from an outsider, why Poland rank that top? I never know people think Poland such OP before reading this thread! Could anyone explain a bit to me?
It's not Poland itself, it's the ideas. You could put Polish ideas on another country and they would still be awesome. 33% cavalry combat ability is pretty great. It means Poland just wipes the floor with anyone they can pull off a flank. They also get inf CA, disc, morale, and manpower, which makes their idea set overall one of the best militarily, I'd argue probably 3rd in the world after Brandenburg/Prussia and Japan. The supporting ideas really shine though. lower accepted cultures can be a big deal, and especially with +3 heretic tolerance too it means you can grab humanist and both ignore heretics and the reformation, and accept any significant culture for a good part of the game.
Brandenburg is obviously being picked because once the ambition is unlocked it is the undisputed strongest military idea set in the game. It's being downvoted because it's not very strong until it gets to there, with some argument that it's also on a country that is likewise not very strong until it gets to there, which is a bad combo. Funny enough I think Brandenburg's ideas would go much better on a different nation, particularly a very large western one, who can afford to deal with some crap to get to space marines.
Ottomans are being picked because of the utility and the order of their ideas. They would be good on almost any country in the game, not just a powerhouse like the Ottomans. Their strength is being able to heavily stack -core costs to obscene levels, and they get tolerance and culture acceptance just like Poland (heathens instead of heretics) so similar arguments to those benefits. It's military ideas are certainly weaker than Poland or Brandenburg, no question, but they're better than most and the discipline is in traditions.