The best list is the one You have most fun with. :-)
Both Poles and Swedes do have some nice special rules making them quite strike capable.
Swedes lack light cavalry but have loads of different firepower.
Poles have nimble shock cavalry that can put a lot of hurt out, but may not handle own casualties at all well.
I have played these at points between 9-12 FP - preferring the larger strenghts.
Muscovy - dirt cheap, can be fierce in defence.
Courland (And Prussia) - have seemed like small defensive armies on skirmish levels.
Rest - I have no playing experience so this is just analysis from trying to decide what to play
Denmark - like Swedes, but less capable.
Cossacks - hordes of infantry and wagons.
Ottomans - I let You know when I get the skirmish force ready. Seem to be a bit like Poles, but not quite as capable, cheaper and more numerous and not quite as agile. And larger forces - like with Muscovy- get bonus for being larger
Imperials - a bit like Swedes. More cavalry oriented. Better equipment, less discipline.