I am using a link to a post from last year for the two Teresas. Both Teresa of Avila and Therese, the Little Flower are Doctors of the Church. Sometimes, Teresa is call the Great Teresa and Therese is called the Little Therese.
They lead us to God more directly than most saints because of the honesty of their writings.
St. Teresa of Avila clarifies the way to perfection for us.
St. Therese of Lisieux gives us an everyday applicable way to holiness. She is perhaps the greatest saint canonized in the 20th Century. Here is my link to what I wrote so I do not have to re-invent the wheel: enjoy.