The Guru gives the title to His student in most of the cases [a]. Sometimes the saint Himself gives Himself a name. Sometimes the disciples give the name. There is no hard and fast rule for this.
Going by pure translation and by definition the highest is Satguru [b]. The Satguru is automatically a Paramhansa, a saint and a Avatar. he is the highest of the highest. He is the indwelling Antaryamin and the Great Parameshwara Himself.
To the devotee of the saint, the title is his expression of love for the saint. When we say 'My dear Sri Sri Sri Maharishi Guru maharaj' it sends a thrill down our spine with the thought of the Guru.
Just going by pure definition and translation, here are some titles and some saints who are called by this title (in no way that means one saint is inferior or superior to the other, it just example of one who is called by that title, that's it all the best)