It's awfully hard to pick one best song from Yuming when she was going by her maiden name of Yumi Arai（荒井由実）in the early 70s. Her voice had been so mellow at that time which especially matched this song "Ano Hi ni Kaeritai"(I Wanna Return to Those Days)...a soft bossa nova-tinged tune talking of a woman's wishes to return to a happier time as opposed to her regret-filled present (probably would've made a great theme song for Mal, the femme fatale character in "Inception"). It was actually the theme song for a TV drama, "Katei no Himitsu" (家庭の秘密....A Household Secret) in 1975, but I think the song has become the much more well-remembered of the two.
"Ano Hi ni Kaeritai", Yuming's 6th single, was released in October 1975 and stayed at the No. 1 spot on Oricon for one week in late December. It had never been a track on an original album but was included in her first Best album, "Yuming Brand"in 1976. Haruomi Hosono（細野晴臣） of YMO fame was on bass for this song, and the backup singer you hear right from the start is Junko Yamamoto（山本潤子）, lead singer for Hi-Fi Set, the trio which often covered Yuming's songs such as "Sotsugyo Shashin"(卒業写真...Graduation Photo).
Listen to this song over a cup of herbal tea. You'll be glad you did.