Beren and Lúthien before Morgoth's Throne in Angband. The battle in song before the throne, from the most important (IMHO of course) of all of Tolkien's works, and the composition that he returned to the most. Why do I say this? On his grave in Wolvercote Cemetery, as well as the details of J.R.R.T. and his wife Edith, are two words, "Beren and Lúthien".
In his eyes the fire to flame was fanned,
and forth he stretched his brazen hand.
Lúthien as shadow shrank aside.
'Not thus, O king! Not thus!' she cried,
'do great lords hark to humble boon!
For ever minstrel hath his tune;
and some are strong and some are soft,
and each would bear his song aloft,
and each a little while be heard,
though rude the note, and light the word.
But Lúthien hath cunning arts
for solace sweet of kingly hearts.
Now hearken!' And her wings she caught
then deftly up, and swift as thought
slipped from his grasp, and wheeling round,
fluttering, before his eyes, she wound
a mazy-wingéd dance, and sped
about his iron-crownéd head.
Suddenly her song began anew;
and soft came dropping like a dew
down from on high in that domed hall
her voice bewildering, magical,
and grew to silver-murmuring streams
pale falling in dark pools in dreams.
She let her flying raiment sweep,
enmeshed with woven spells of sleep,
as round the dark void she ranged and reeled.
From wall to wall she turned and wheeled
in dance such as never Elf nor fay
before devised, nor since that day;
than swallow swifter, than flittermouse
in dying light round darkened house
more silken-soft, more strange and fair
than slyphine maidens of the Air
whose wings in Varda's heavenly hall
in rhythmic movement heat and fall.
Down crumpled Orc, and Balrog proud;
all eyes were quenched, all heads were bowed;
the fires of heart and maw were stilled,
and ever like a bird she thrilled
above a lightless world forlorn
in ecstasy enchanted borne.
All eyes were quenched, save those that glared
in Morgoth's lowering brows, and stared
in slowly wandering wonder round,
and slow were in enchantment bound.
Their will wavered, and their fire failed,
and as beneath his brows they paled
the Silmarils like stars were kindled
that in the reek of Earth had dwindled
escaping upwards clear to shine,
glistening marvellous in heaven's mine.
Then flaring suddenly they fell,
down, down upon the floors of hell.
The dark and mighty head was bowed;
like mountain-top beneath a cloud
the shoulders foundered, the vast form
crashed, as in overwhelming storm
huge cliffs in ruin slide and fall;
and prone lay Morgoth in his hall.
His crown there rolled upon the ground,
a wheel of thunder; then all sound
died, and a silence grew as deep
as were the heart of Earth asleep.
Beneath the vast and empty throne
the adders lay like twisted stone,
the wolves like corpses foul were strewn;
and there lay Beren deep in swoon:
no thought, no dream nor shadow blind
moved in the darkness of his mind.
'Come forth, come forth! The hour hath knelled,
and Angband's mighty lord is felled!
The Geste of Beren and Lúthien
(Lines 4,044 to 4,115)