Yeah. I assume what the OP means by "classic rock," by genre, is pre-punk, pre-college rock (which eventually morphed into what we understand as "indie": art-rock, alt-rock, and various post-punk offshoots). For easy reference pre-80s. So I can't pick Bela Lugosi's Dead(otherwise I would), even though it's an old song by 1979, since Bauhaus are decidedly gothic-rock & post-punk.

Most of my favorite rock genres are decidedly after classic rock, so this requires some thinking. I think I'll go with:

"Time" -- Pink Floyd, a song whose lyrics, unlike a lot of songs you latch onto in one's youth, become more resonant with age.

"Venus in Furs" -- Velvet Underground. The Strokes and Sonic Youth owe everything to VU.

"I wanna be your Dog" -- The Stooges. Meet your maker, Jack White.

"Hey Joe." -- Popularized by too many artists too count. Take your pick: The Leaves, the Byrds, Tim Rose, The Standells. And oh, that little known chap Jimi Hendrix.

I have to say, I prefer the Austrian version, Heast Franz. :P

Mafia 2 theme song.. In fact some stuff but I don't remember nothing.

1. Baby Come Back

2. More Than a Feeling

Not sure if I have heard all versions, but have to go with Jimi on that one.
As a metal head, I hate myself for liking Boston, but I do enjoy More than a Feeling and other songs by them.

Phoenix - Delfinul, dulce dulful nostru (1975)

Phoenix,

One of the greatest Romanian classic rock bands, that rivals easily worldwide knows names (such as Little Feat, King Crimson, Captain Beefheart, Cactus, etc)

I saw the stooges live a couple of years ago at a really lame music festival. They had more energy and rock'n'roll in them than all of those other young bands of 20 year olds put together. I waited a whole day listening to mediocre rock bands for them and it was so worth it.

Enter Sandman

Bring it on Home by Led Zeppelin. By the band that pioneered what we now consider modern rock/metal to be, this song is a two-part masterpiece that bridges classic blues and modern blues rock.

Bohemian Rhapsody

I'd do anything for Love

We can only hope they qualify. It's nice you brought up "Ace of Spades", since I was about to do it myself, and "Iron Man" by Black Sabbath.

ELO - Wild West Hero

Supertramp - Goodbye Stranger

The Kinks - Lola

Styx - The Grand Illusion

Derek and the Dominos - Layla

The Real Me - The Who

Love Reign O'er Me - The Who

Hey You - Pink Floyd

Comfortably Numb - Pink Floyd

The Wizard - Uriah Heep

Stairway To Heaven - Led Zeppelin

Rockin' All Over The World - Status Quo

Sledge Hammer - Peter Gabriel

Bohemian Rhapsody - Queen

Hammer To Fall - Queen

Don't Fear (The Reaper) - Blue Öyster Cult

Godzilla - Blue Öyster Cult

All You Zombies - The Hooters

Johnny B - The Hooters

You're The Voice - John Farnham (Yeah, it's Pop-Rock, but I still love it.)

Rebel Yell - Billy Idol

White Wedding (Part 1) - Billy Idol

The Bard's Song (In The Forest) - Blind Guardian

Tubular Bells - Mike Oldfield

Jeff Wayne's Musical Version of "The War Of The Worlds"

Well, Tubular Bells may technically not count, as it is not a song, but a concept album. It is also not entirely Rock, but a melange of several genres. However, it is a classic (just turned 40 last Saturday), and my favourite album of all time, so it had to go on this list.

Jeff Wayne's "The War Of The Worlds" is my second favourite album, based on my second favourite book. It is a concept album that also happens to be a audiobook and a Rock Musical, so it may not count either. I also have a hard time choosing among the songs, because I pretty much love all of them. Well, except for The Red Weed (Part 1). However, if I absolutely had to choose, I would probably go with Brave New World.

Fail.