In this post I'll try to convince you that the Poland is the best pick, but I'll be objective. I'd be extremally happy if Poland manages to make it to MkIII.
To start up, Poland never stood a chance in MkII, because there were too many civs in Europe. They should have their chance to redeem themselves.
Sigismund's Poland is the worst pick, I wouldn't recommend voting for it. With not useful UB and UA, they are only left with Winged Hussar and that's not too much to be honest.
Sobieski's Poland is much better, with battle oriented UA that basically gives all your Cavalry Units Winged Hussar's ability. However, rest of their uniques aren't good enough to justify a vote.
Jagiełło's Poland is really solid pick in my opinion. They get bonus production towards creating military units AND said units start with more EXP. That will help edge out their neighbours. His Knight replacement fights better when next to Knights from the other cities. The thing that makes the civ shine is their Workshop replacement. It heals random Polish unit at the start of the turn. That's really good, imagine having 5 Workshops. When sieging a city you would have massive advantage.
The best reason to vote for Poland is Piłsudski. They get bonus for fightin against a stronger enemy and let's be honest, Russian civ will for sure be stronger than any Central European civ. After Industrialisation, Workers can build a bit weaker anufactories and Custom Houses (-1 yeild if I recall correctly). You know what does it mean? A crap ton of Production for Poland (and Gold too, which is more significant with Smart AI)! We didn't even go to UUs yet - PZL.23 is Fighter replacement, that actually can be used by AI. In my experience, AI tends to sometimes spam Fighters and after all it's not using them properly. PZL.23 has much more attack against land units and has bigger range - it can be used to efficientally bombard land units, instead of just pinging cities that are already in black. The real MVP is Legionary Uhlan (Cavalry replacement) - attacks twice (great!) and deals damage to all nearby units after killing an enemy. That's not everything - it obsoletes later, as it gets stronger with time. Even in Atomic Era this guys are still viable! That's one of the longest existing
Other reasons:
* Memez
* I will love you if you vote for them
* better than any other civ to choose
* Pilsudski tends to do very good in my personal games
* Cavalry and Horses are awesome

Make Poland great again! Make my dream happen! Vote Poland (Pilsudski)!

Lefondesin