The greatest war happened in Satya yuga and arguably, most devastating war. It was the Tarakamaya war, Ramayana and Mahabharata pales in comparison to this.
The conflict occured after Soma abducted Tara, wife of Brihaspati. Despite the devas requesting him to return her, Soma did not relent, Gods headed by Shiva, Vishnu and Indra launched an assault on Soma who was assisted by Shukracharya(due to enmity with Brihaspati) and his demon hordes.
As per puranas, in this battle, Vishnu killed a demon named Kalanemi as well as several others, whilst Shiva hurled Brahmashira against demons. , Shukracharya was the main adversary of Siva in this battle.
The war went for a long time with either side undefeated,. A group of devas requested Brahma to put an end to it. Brahma's intervention concluded the war and Tara was restored to Brihaspati.
He disregarded the sons of Aṅgiras and suddenly abducted the renowned wife of Bṛhaspati named Tārā.
30. Even after being requested by the Devas and divine sages, he never let-off Tārā to Bṛhaspati, the son of Aṅgiras.
31-33. Uśanas took up his (Soma’s) side and Bhava (Śiva) that of Aṅgiras (i.e. Bṛhaspati), for that person of great brilliance was the disciple of the father of Bṛhaspati formerly. It was out of this affection that lord Rudra became the ally of Bṛhaspati and took the bow Ajagava.
A great miraculous missile named Brahmaśiras was discharged by that noble-souled one after aiming at the Devas, whereby their reputation was destroyed.
34. There a well-known battle between the Devas and Dānavas took place. It was known as Tārakāmaya[2] (concerning Tārā). It was destructive of the worlds.
35. The Devas who survived this battle are remembered as Tuṣitas. They sought refuge in god Brahmā, the primordial lord, the grandfather of the world.
36. Then the grandfather (i.e. Brahmā) himself came there and prevented Uśanas and Rudra, from fighting. He restored Tārā to Aṅgiras (Bṛhaspati).
~Brahmanda Purana.
O dear one, formerly Bṛhaspati’s glorious wife named Tārā was forcibly kidnapped by the powerful Candra (i.e. the Moon). Kidnapping Bṛhaspati’s wife the Moon took her to his house and dallied with her for a long time. O dear one, after some time she conceived. Then Bṛhaspati appealed (to Candra to return) his wife. Candra too, overcome by pride and puffed up by his power, did not give her (back). Then, O dear one, Bṛhaspati, getting ready along with gods like Indra, started fighting with the powerful Candra. To help Candra Śukra then came there to the battle that was commenced with the demons. Then a great war for Tārā took place. All people will look upon (that battle called) Tārakāmaya as important. In that very terrible war (both) gods and demons were killed. O dear one, no one was victorious or defeated. Then Brahmā came there, and stopping that fierce battle, gave, after admonishing the Moon, Tārā to Bṛhaspati. Bṛhaspati, seeing Tārā to be pregnant, was angry, and in the presence of Brahmā (said to) Tārā in the gathering of the gods and demons.
~ Padma Purana
Having disregarded Vrihaspati, the son of Angiras, the moon by force carried away his illustrious wife Tara.
31. And albeit solicited by the gods and Rishis he did not return her. Vrihaspati, the the preceptor of the gods, was accordingly enraged with him.
32-33. Ushana (Suka) began to protect his sides. The illustrious deity Rudra was formerly a disciple of Vrihaspati’s father. Out of reverence for him he (Rudra) protected his rear, taking his Ajagava bow in his hands.
34-35. A terrible battle, destructive of the gods and demons on account of Tara, was fought at the place where the high-souled Siva discharged his mighty weapon Brahmasiras—for the destruction of the demons and where their glory was (accordingly) destroyed.
36. O descendant of Bharata, in that battle the gods, well read in the precepts of religion who had sided Vrishaspati, and the deities Tushitas, who belonged to Soma’s party who had stolen away his preceptor’s wife, sought refuge with the Prime Deity the Eternal Brahma.
37. Thereupon the Grand-father himself came there, prevented Sukra and Sankara (to fight) and gave back Tara to Vrihaspati.
~Harivamsa purana.
It's not given in detail in Mahabharata, only parts of it are mentioned.
Indeed, it was upon that car that Indra and Vishnu had fought of old in the battle (with the Asuras) in which Taraka (the wife of Vrihaspati) had become the immediate cause of much slaughter.
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I put this armour on thy body, joining its pieces with the aid of Brahma strings. In days of yore, Brahma himself had thus put it on Vishnu in battle. Even as Brahma himself had put this celestial armour on Sakra in the battle caused by the abduction of Taraka.
