One where absolutely everything is perfect - the beat, the melody, the arrangement, the structure, the lyrics, the subject, etc.?
Iron Maiden - Hallowed Be Thy Name is what I consider quintessential Maiden. Everything from Harris’ bass to Dickinson’s incredible vocals.
While I was initially going to put Black Sabbath (the song) here as well, I think War Pigs is far better option.
The composition, the lyrics, Ozzy’s voice, and every instrument feeling distinct and getting its own little bit of spotlight. It’s just a fantastic song in so many respects.
There are a few others, but I’m just sticking with those two for now.
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.