I go to pray to a mosque which is very close to my house (take about 3 minutes). So usually I pray Nafil prayers before Iqama at home and go to mosque for the Jamaah. Could you tell me the best way to do it, at home or in the mosque?

Praise be to Allah, the Lord of the World; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

It is better to pray Nafila in the home than to pray it in the mosque. There are many Hadith proving this. al-Bukhari and Muslim reported that the Prophet (Sallallahu Alaihi wa Sallam) said: "O people! Perform your prayers at your house, for the best prayer of a person is what is performed at his home except the compulsory prayer" .
al-Bukhari also reported that the Prophet (Sallallahu Alaihi wa Sallam) said: "Offer some of your prayers (Nawafil) at home, and do not take your houses as graves" .
Ibn al-Sakan reported from Dhamrah from his father that the Prophet (Sallallahu Alaihi wa Sallam) said: "The prayer of a man in congregation is twenty-five times more superior to his prayer offered individually, and the voluntary prayer that is performed in his house is as superior as the prayer performed in congregation compared to the prayer performed individually" [Imam al-Albani graded it sound].
Allah knows best.