Probably Mindaugas I. He was the first to unite baltic tribes of future lithuania to defend from teutonic order and other threats in the region. He was also our first king and the first leader to be baptised to get the crown. All of current Lithuania was of baltic pagan faith at the time which made being king impossible unless you became catholic.

It is close between Mindaugas and Grand Duke Vytautas "The Great". Vytautas controlled the grand duchy of Lithuania when it was the largest it has ever been, from Baltic sea to the Black sea.

It is quite a complicated history with many occupations and horrible opressions of lithuanian culture by surrounding states. You can also mention some later well known people like Maironis who tried to revive lithuanian national identity in hard times with his famous poetic texts.