St. Elizabeth Ann Seton, as has been mentioned already. She is my patron saint for my middle name (Elizabeth), and I now work for the Catholic Schools office in our diocese, so I appreciate her story even more.
Blessed Stanley Rother, who was martyred in Guatemala during the Guatemalan Civil War. He has an impressive story too. Not so book smart in seminary, but he managed to learn the Mayan language of the people he served. He left Guatemala briefly during the Civil War, but returned, even though he knew his life was threatened, saying a shepherd cannot leave his sheep. He was killed a few months later. His body was brought back to Oklahoma, but his heart was buried in the Guatemalan community where he had served.