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.