I see Harry knows what's best. I was testing vodka actually.
There are other great brands, but this one is really good.
Here I found some random wholesale prices:
Wyborowa, 0.5 litre - around 17 zł, but why not buy bigger bottles.
Also safe choice: Luksusowa, Żołądkowa Gorzka.
And do not buy some bad brand, the quests (and you) may have a nasty hangover or at least bad memories from the reception after not genuine brands like Absolwent [suprisingly popular, but suspiciously similar looks to Smirnoff, wonder why? ;))], Lodowa, Sieradzka...