For me, probably a Beatles song, "Yesterday," "Hey Jude," or "Let It Be."
Those songs were separated by some five years. Surely you can remember something a bit closer to those last two than "Yesterday." Most stations weren't playing five-year-old Beatles songs in 1970 -- what oldies did get played on Top 40 radio were mostly pre-British Invasion, and there was so much current material moving up and down the chart that recurrents weren't often programmed. So how do you think you might have heard "Yesterday" new, or if not that, then nothing at all until 1969 or 1970?