What is, in your opinion, the perfect song or track?

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.

Thanks, I would have never stumbled upon this song, but it gave me goosebumps. Going on my playlist.
I would have never thought of Unwritten for this sort of question but after reading your post … you’re 100% right.

Simon and Garfunkel: The Boxer

M83 - Midnight City

God Only Knows by The Beach Boys

The arrangement and instrumentation are incredible.

Totally good choice. I really need to dig deeper in their work. I love Good Vibrations so much too.
- Pet Sounds is Brian Wilson’s magnum opus and a great place to start, but beyond that it gets…complicated. After Pet Sounds he started work on an even more ambitious album, Smile (which is where Good Vibrations came from). This produced dozens of hours of content but the album was shelved, unfinished, when he wasn’t able to put it all together due to his worsening paranoia / schizophrenia.
  
  After that album fell apart they released a series of albums in the 70s that were composed of stripped down versions of songs originally meant for Smile (like Vegetables and Surf’s Up) alongside compositions by other Beach Boys members and the occasional new material from an ailing Brian Wilson. Those albums have some absolute gems (like “Til I Die” and “Time to Get Alone”, which both give you a peek into where his mind was at in the 70s) but are very very hit and miss. Surf’s Up and Smiley Smile are probably my favorites as complete albums.
  
  The Smile Sessions was released in 2011 which includes an approximation of the full Smile album as it was intended. I say “approximation” because it’s still pretty clearly unfinished in spots, but there are some stunning compositions in there and overall you can see what he was going for. It also includes hours of studio sessions and instrumentals which can be really interesting to listen to. I find the instrumental tracks from Good Vibrations really neat - tons of sections that sound awesome but ended up being cut.
  
  Brian Wilson also released a “completed” Smile under his name in the 2000s, but I don’t enjoy it as much because (a) it still feels unfinished (b) some of the arrangements feel worse than the original and (c) his voice just has not aged well.
  
  Then there’s their earlier stuff which is pop Americana but if you’re into that it’s honestly quite good.

@KISSmyOS I’m gonna go with Strawberry Fields Forever.

Definitive classic
- I am so fucking glad that Dylan introduced the Beatles to weed.

Radiohead - The Bends

Demanufacture, the opening track on Fear Factory's second album. Perfect tone, buildup and release, and aggression.

It's a bit of cheating, as it has nine parts, but Shine On You Crazy Diamond. Perfect instrumental, perfect lyrics, perfect everything.

As a perhaps distant second place, To nie Ptak. The instrumental is at the same time melancholic, sultry, and dreamy, and Kayah, well, she's an amazing singer. I love how this song builds a metaphor through its denial ("she isn't a bird of paradise", implying "she's a woman"), only to contradict it near the end (when darkness steals your heart, you'll see that she's a black-feathered raven).

If you are hippie-souled like me then that track would be

Renaissance: Ashes are Burning

Warning, it is a long track and recorded before many advances in studio equipment but every second of it is wonderful to me.

“Your Song” — Elton John

Cortez the Killer - Neil Young

Maggot Brain - Funkadelic

The guitar play is sublime. If I’m sitting around partaking in certain recreational activities, these two songs are on the list.

I have 2 very different ones that I think couldn't be improved:

Queen – Bohemian Rhapsody
Seeed – Molotov

Bruce Springsteen - Jungleland

Nightcall by Kavinsky.

Every part of it, the melodies, rhythm, vocals, is very simple and straightforward. But it's much more than the sun of its parts. It's catchy enough that most people I know really like the song. But it's strange enough to introduce a lot of those people to new sounds and ideas.

Totally agree. That song is so awesome.

It might be a bit of a cliché, but my pick would be Teenage Kicks by the Undertones. It's such a perfect representation of its times, there's a longing in the lyrics that I still feel every time I hear the song, and while not technically a masterpiece in terms of depth or skills, there's nothing you could add or change that would make the song any better.

Or if a partial song counts, there's of course that guitar solo by Prince around the 03:25 mark.

Transcendental Youth - The Mountain Goats

Title track and closer of one of my all-time favorite albums. The horns are just perfect and the lyrics are beautiful. Makes me think of those little moments of getting lost in something good during bad times, a short escape you know won't last.