Champions Bar at the Warsaw Marriot does excellent burgers. At a price.
A very very high price from what I remember. Anf I must say that none of the food I've ever had there was any good.
The TGI restaurant in Galeria Malta in Poznań do pretty decent burgers
The one in Warsaw is pretty hit or miss. I have had a very decent burger there but I have also had charred rubbish. One time I had a bacon cheeseburger and the bacon was completely uncooked in the middle but charred at both ends. Both pieces were like that!
The most consistent burgers are at Bar Below but the Mexican burger at Warsaw Tortilla factory is better when it is good.
Personally, I just make my own burgers when I want good ones.