If were including historical characters.
Then theirs no surprise as to who my greatest Cossack ( Ops I mean greatest Ukrainian ) is going to be - none other than Taras Bulba.
A legendary character made famous by the russian author Nikolai Gogol - you probably recall the film where Taras and his cossacks - after helping the Poles repel the Ottoman invasion of Ukraine - is betrayed by the Polish king who turns his cannon on the cossacks. Swearing revenge against the Poles, Taras chops off the Polish kings hand with his sword and vows to drive the Poles out of Ukraine.
So begins the long bitter struggle against the Poles with tales of love and betrayal as one of Taras's sons is seduced by the daughter of the Polish governor of Dubno.
Anyway a great ripping yarn of daring do with those vagabonds of the Steppes in the 17th century.

Taras Bulba on the net:
http://en.wikipedia.org/wiki/Taras_Bulba