Shmuel ha'Katan was the holiest person of his generation. This is heavily implied by the Gemara in Sanhedrin (11a). The Gemara states that when the sages were gathered together, a Heavenly voice rang out and stated, "There is one person amongst you who is fitting to have the Shechinah rest on him, but his generation is not worthy of meriting such a thing." The Gemara states that everyone present looked at Shmuel ha'Katan, indicating that it was clear that the Heavenly voice was referring to him.

Why was such a holy person needed to form this Berachah? Many commentaries, including the Pnei Yehoshua and Ben Yehoyada, explain that every Berachah of Shemoneh Esrei was formed by the Anshei Knesses ha'Gedolah in a way that very intricate details, such as the amount of letters and words in the Berachah, have a tremendous impact on the acceptance and power of the Tefilah. In order that this Berachah should also have a similar power to that of the other eighteen Berachos, it made sense that the holiest person who was closest to having the Shechinah rest on him would be the best candidate to formulate this Berachah in this manner. [This explanation is part of a longer explanation by the Ben Yehoyada.]

All the best,

Yaakov Montrose