Narada Muni was well known for his devotion towards Lord
Vishnu. He always used to chant "Narayana Narayana" (Narayana
is one of the names of Lord Vishnu). He used to think that he
is the greatest devotee of the Lord. Once upon a time he went
to Vaikuntha, the abode of Lord Vishnu. During their
conversation Narada intentionally asked - " O Lord! You have
so many devotees. I am curious to know who is your favorite
one." Narada was expecting that Lord Vishnu will honor him as
his favorite devotee. To his shock Vishnu pointed to a poor
farmer on the earth and said "That poor farmer is my favorite
devotee."
Complete story can be read
here.