Biggest lamp with a Brightest Flame : The Karthigai Deepam. The Karthigai carnival :

As an annual festival feature, when  the moon shows its whole disc illuminated in the month of Karthigai the celebration becomes a week long temple function. A big brass vessel especially designed in the form of a lamp is lifted to the peak of the hill; it is filled with tins and tins of ghee and pots and pots of butter; and an enormously thick wick made of loosely twisted woven fibers (clothes) is immersed into the vessel and one end of the wick is drawn out  to the apex and is ignited with burning camphor and the thick wick fully soaked in ghee acting by capillary attraction  conveys  the  ghee the main  illuminant  to  the flame around  6 pm when the sunlit full  moon  disc emerges from the east and comes on to the horizon. The luminous glow of the lamp atop the hill opens the eyes of millions of people to something divine and auspicious and ardent devotees and others see in the burning light lord of the world and remain visibly, moved with their mouths uttering "Om Arunachaleshwaraya Namaha", Here man lights the largest wick of the largest lamp once a year and the world goes into rapture sings his songs of grace.

Deepam - The Symbol of Lord Arunachala  

History has is that the practice came into existence following a dispute between Lord Vishnu and Lord Brahma over who was the superior of the two after the some hindrance to their attempts to gauge their strength, the two prayed to Lord Siva for his Grace.

 Their test was to find the top and the bottom of a blazing mount which was actually Siva. Lord Siva blessed lord Vishnu and Lord Brahma and said that he would appear as a Jothi (flame) on top of the hill. It is to perpetuate this event that the flame is lit on top of the holy hill during the month of Karthigai (November - December).

This festival begins in the Uthiradam (Star) day in Karthigai and winds up with the Bharani Deepam lit in the early hours of the tenth day. The Deepam is lit at dusk, coinciding with a Full Moon. The Deepam can be seen even from a distance of 45 Km. Real image of lord Arunachala is viewed in the appearance of "Agni". This is celebrated every year as the remembrance of one of the image of lord Arunachala as fire. During the holy day, more than 10,00,000 devotees get this heavenly worship of Lord.