One where absolutely everything is perfect - the beat, the melody, the arrangement, the structure, the lyrics, the subject, etc.?
I have 2 very different ones that I think couldn’t be improved:
Unwritten - Natasha Bedingfield
It’s a banger and works in any situation. Dance floor? Everyone’s singing and bopping. Alone in your room depressed as fuck? You’re cry-singing your heart out. Wedding? Lovely song about starting a new chapter together. Working out? It’s a motivational masterpiece. It’s so versatile and so good.