Shiv-Shakti
Shiv is Parabrahma and Shakti is Parashakti and they both are the source of creation in the Universe. They both are parents of the entire universe including atoms, rays, light, matter, waves, tissues, etc. They are the first couple in the universe. Shiv is nothing without Shakti. Shiv needs Shakti to live , to provide creation and destruction of the world. When she was Sati, she married Shiv but self immolated herself when she saw her father Prajapati Daksha insulting her and her husband. Then she came back as Shiv and Shakti are incomplete without each other and as Parvati she reunited with Shiv.
Shri Ram and Mata Sita
They are not mythology but actual history still they are the most beloved couple because they were entirely devoted towards each other. There was no one for them except each other. Shri Ram was a King yet he refused to be polygamous and never even maintained a harem. His life was empty without his beloved wife. Uttar kand is an interpretation but even if her exile happened, still he didn't abandon her as his wife. He refused to marry anyone else even when he was pressurised for it and when Mata Sita left the world, he made her golden statue and did penances and Yajnas and led his life as a celibate caring for their twis Lava and Kusha.
Radha Krishna
Radha is eternal consort of Shri Krishna in Goloka and in mortal world she was born as daughter of Vrishbhanu and Kalavati (Kirtida) and Brahma married them in Bhandirvan. Despite various regional folklores and other traditions, originally Radha is the wife of Krishna and she emerged from him so she is also his Shakti. They are incomplete without each other. He left her and other Gopis in Vrindavan when he left for Mathura.
Radha is source of all the wives of Shri Krishna as when he is Ram she is Sita, when he is Vishnu she is Laxmi, when he is Parbrahma (Vishwaroop) she is Parashakti.
Kusha and Kumudvati
Kumudvati was a Naga Princess and Kusha was son of Shri Ram and therefore benefactor of Garuda, enemy of Nagas. Garuda had killed Kumudvati's father. Yet, their love passed all the obstacles as it was a divine plan. Rishi Agastya prophesized that his first wife Champika won't have sons but daughters and he expected advent of Kumudvati. Later, after thinking that Nagas had taken his golden arm- bracelet when he was sporting in Sarayu with Champika and other junior wives and almost annihilating them with Garudastra, Naga King Kumud came out along with Kumudvati and returned the bracelet and gave her hand in marriage to Kusha. She was his favourite wife and he built Nageshwarnath Temple just for her. She followed Kusha in death as she couldn't live without him. They are eternal consorts living peacefully in heaven together forever.