Hmm love these threads and seeing what people think - a good chance to expand your tastes
Many, many contenders from Pink Floyd - wish you were here, comfortably numb, and 'time / breathe reprise / great gig in the sky' (in a row - the latter always gets me a little misty eyed!).
I probably listen to Metallica more than any other band and I think 'One' is an absolute masterpiece. 'Blackened' is also a joint favourite (but that might be a bit too heavy for some people here).
These are some other select songs I have in a playlist called 'best of the best':
Radiohead - Paranoid Android
Beastie Boys - Sounds of Science (also makes for a fun 'spot the beatles sample' game)
Lou Reed - Perfect Day
Del Shannon - Runaway (adore that one)
Radiohead - Killing in the Name
Tracy Chapman - Fast Car
Eagles - Hotel California
Outkast - B.O.B. (always makes me smile)
Dire straights - Sultans of swing
Beatles - Help
AC/DC - Shoot to thrill
Kids in America - Kim Wilde
Everlong - Foo Fighters
Deadmau5 - I remember (another one that I find quite emotional for some reason - makes me think back of my past fondly)
Beethoven - Moonlight Sonata (the final movement is the bestest)
My tastes are pretty mainstream but I think that covers relatively wide ground... must suck not to enjoy a variety!