Wrocław 🙂 We’re there 3/4 times a year. Not as many tourist attractions, but nice and easy to get around, plenty of nice restaurants etc. If you’re going there with kids, I recommend the zoo - one of the best (better than London!) and the waterpark near the university is good. During the summer they have some great bars in the parks run out of shopping containers (no drunken louts, just sitting out chatting and nice background music).
Poznan I’ve been to half a dozen times as BIL and his family are there, mostly in winter - Christmas markets are good, old town is ok, but not a huge amount of tourist stuff.
Gdańsk I like better in the summer (but Christmas again is nice). Lots of walking by the harbour, bit of history (WW2 etc) but we usually head out to enjoy the summer near Hel or Utska. They have a great music festival each year, Opener, in Gdynia. I’ve been only once but nephews love it - actually gets good acts (Strokes, 1975, Nick Cave, Smashing Pumpkins, Kylie, Radiohead, Artic Monkeys etc).
Szczecin is pleasant but boring. Not a lot to see, place somehow seems deserted, very clean, but boring. Only been once, wouldn’t rush back.
I’ve not been to Warsaw or Łódź but DH is Polish so I’ll get there eventually.