(Note : I won’t judge either son of Fëanor based on their actions in the Kinslaying, as they were all under their father’s Oath)
The oldest son of Fëanor is together with Maglor the most likeable of the 7 brothers, as together with Maglor he was most alike to his mother in spirit – Nerdanel the Wise, who was « more patient than Fëanor, desiring to understand minds rather than to master them, and at first she restrained him when the fire of his heart grew too hot; but his later deeds grieved her, and they became estranged. » Just like his mother, he too always tried to restrain his brothers’ violent temper.
Maedhros is also the only son of Fëanor who befriended someone from outside his own House : Fingon (Findekano), the valiant son of Fingolfin. Therefor, when Fëanor betrayed Fingolfin by burning the ships at Losgar, Maedhros was the only one who protested as his heart went out to his friend :
But when they were landed, Maedhros the eldest of his sons, and on a time the friend of Fingon ere Morgoth's lies came between, spoke to Fëanor, saying: 'Now what ships and rowers will you spare to return, and whom shall they bear hither first? Fingon the valiant?'
Then Fëanor laughed as one fey, and he cried: 'None and none! What I have left behind I count now no loss; needless baggage on the road it has proved. Let those that cursed my name, curse me still, and whine their way back to the cages of the Valar! Let the ships burn!' Then Maedhros alone stood aside, but Fëanor caused fire to be set to the white ships of the Teleri. So in that place which was called Losgar at the outlet of the Firth of Drengist ended the fairest vessels that ever sailed the sea, in a great burning, bright and terrible. And Fingolfin and his people saw the light afar off, red beneath the clouds; and they knew that they were betrayed. This was the first fruits of the Kinslaying and the Doom of the Noldor.
(from Chapter 9 of the Quenta Silmarillion)
Maedhros was also wise and noble, as after he was saved from his ordeal in Thangorodrim by Fingon – which cost him his right hand – he sought to end the feuds between the Houses his father had started, as he was aware that in order to defeat Morgoth, all Noldor should be reunited ; to do this, he passed the High Kingship back to Fingolfin, something neither of his brothers would have even considered :
There Maedhros in time was healed; for the fire of life was hot within him, and his strength was of the ancient world, such as those possessed who were nurtured in Valinor. His body recovered from his torment and became hale, but the shadow of his pain was in his heart; and he lived to wield his sword with left hand more deadly than his right had been. By this deed Fingon won great renown, and all the Noldor praised him; and the hatred between the houses of Fingolfin and Fëanor was assuaged. For Maedhros begged forgiveness for the desertion in Araman; and he waived his claim to kingship over all the Noldor, saying to Fingolfin: 'If there lay no grievance between us, lord, still the kingship would rightly come to you, the eldest here of the house of Finwë, and not the least wise.' But to this his brothers did not all in their hearts agree.
He was also a valiant warlord, as during the Dagor Bragollach he was the only one amongst the sons of Fëanor who wasn’t force to retreat and flee : he kept the fortress on the Hill of Himring and closed the Pass of Aglon.
On the whole, Maedhros could’ve easily been a son of Fingolfin – maybe he should’ve been adopted ?
Maglor (Canafinwë ; Macalaurë)
The son with the golden voice is together with his brother Maedhros the most likable ; he is most renowned as a lover of poetry and music, yet he was a proficient warrior as well as during the Dagor Bragollach he didn’t flee like the rest of his kin, but instead joined his brother Maedhros, with whom he remained til the (bitter) end.
Furthermore, Maglor took pity on Elrond and Elros during the third Kinslaying and convinced Maedhros to spare them (which proved to be quite useful in the coming Ages) : he either raised them by his own and began to love them (in the Sil) or left them in a cave near a waterfall (in UT) where they were later on located by their mother’s people.
Like his brother, Maedhros seemed a wise, noble and kind man who had no place in his father’s quest, yet swore the Oath and was forced to do his share until the end.
In the end, Maglor sought to make the Oath void and submit to Eönwë, but was ultimately persuaded by his brother, who also loathed the Oath in the end, but feared the Everlasting Darkness they had taken upon themselves if they’d not fulfill their Oath.
Celegorm (Turcafinwë ; Tyelcormo )
His mother named him Tyelcormo because of his impetuousness and violent temper (and that’s really saying something).
He was a great hunter back in Valinor, and named as a friend of Ulmo’s (of whom he received Húan).
He was always together with his brother Curufin, and after messing up and fleeing during the Dagor Bragollach, he joined the ranks of Finrod Felagund in Nargothrond, where together with his brother he tried to sabotage Finrod as much as possible, and even tried to usurp him :
But the curse of Mandos came upon the brothers, and dark thoughts arose in their hearts, thinking to send forth Felagund alone to his death, and to usurp, it might be, the throne of Nargothrond; for they were of the eldest line of the princes of the Noldor.
(from the story of Beren and Lúthien)
Together with his brother, he’s also known for his ignoble actions in the story of Beren and Lúthien :
not only did he refuse to help Beren and Finrod when they were in Sauron’s captivity, as they wanted them to die so they could usurp him, but he also held Lúthien captive and wanted to force Thingol to give him her hand --- yes, he was a true romantic.
Then, when the brothers’ plans backfired and Beren and Lúthien were together again, they espied the couple and Celegorm tried to run over Beren with his horse, whilst his brother captured Lúthien – which again backfired, after which the brothers again fled in shame.
Celegorm is arguably the worst son of Fëanor ; together with his brother he was most filled with dishonour and malice, and was responsible for the most loathsome and cowardly actions.
Caranthir ( ? ; Carnistir)
Albeit his alias ‘the Dark’, I don’t regard him as the darkest son of Fëanor at all – he’s regarded as the son who was most quick to anger, and the harshest one, which can be understood from this quote :
But Caranthir, who loved not the sons of Finarfin, and was the harshest of the brothers and the most quick to anger, cried aloud: 'Yea more! Let not the sons of Finarfin run hither and thither with their tales to this Dark Elf in his caves! Who made them our spokesmen to deal with him? And though they be come indeed to Beleriand, let them not so swiftly forget that their father is a lord of the Noldor, though their mother be of other kin.'
(from Chapter 13 of the Quenta Silmarillion)
He and his people were the first to meet the Dwarves, and despite a difficult start they started a relationship that benefited both.
Furthermore, Caranthir saved the Haladin from the Orcs and looked kindly upon them, as he sensed the valour of them and their leader, Haleth.
To me, Caranthir is certainly not the worst son of Fëanor, although he had a violent temper, he was valiant and noble and never treacherous, unlike Celegorm and Curufin.
Curufin ( Curufinwë ; Atarinkë)
Curufin was most alike to his father in temper, looks and skill (that’s why he received the same name like his father), and also his father’s favourite.
I regard Curufin only less worse than Celegorm, because Curufin actually had some skills in the field of craftsmanship, which he passed over to his wise son Celebrimbor, who would become such an important figure in the history of Arda.
It must be said though, that Curufin rightly chastised Eöl for how he handled a Lady of the Noldor :
'Do not flaunt the title of your wife before me,' he said. 'For those who steal the daughters of the Noldor and wed them without gift or leave do not gain kinship with their kin. I have given you leave to go. Take it, and be gone. By the laws of the Eldar I may not slay you at this time. And this counsel I add: return now to your dwelling in the darkness of Nan El-moth; for my heart warns me that if you now pursue those who love you no more, never will you return thither.'
(from Chapter 16 of the Quenta Silmarillion)
Amrod ( Pityafinwë ; Ambarussa)
Not much is known about Amrod and his twin-brother, safe that they were great hunters and had reddish hair. Amrod died in the mouths of Sirion, during the third Kinslaying.
In the Dagor Bragollach, he and his brother sided with the Laiquendi and withstood the forces of Morgoth. Nor he nor his brother seemed to have done anything malicious, safe joining in the Kinslaying like all of them.
Amras (Telufinwë ; Umbarto/Ambarto)
The youngest son of Fëanor was the favourite of his mother (not surprising, since he was the youngest one of the lot) and she desired that he stayed with her instead of going to Beleriand, but Fëanor refused. Umbarto, the Fated, accidentally died during the ship burning at Losgar, although in the Sil he dies together with his brother in the mouths of Sirion.