Ok this is so difficult so I'm just gonna list 10 favourites of mine at the moment (but not of all time because that is just uber impossible)
In no order:
Pink Floyd - Shine On You Crazy Diamond (awesome Floyd song)
Eric Clapton - Tears In Heaven (beautiful song)
Oasis - Don't Look Back In Anger (classic song from the best band ever)
Nirvana - Smells Like Teen Spirit (great song even though I hate this type of music)
The Verve - Bittersweet Symphony (iconic song and genius lyrics from Ashcroft)
Jimi Hendrix - Little Wing (best Hendrix song in my opinion)
Queen - Bohemian Rhapsody (brilliant song from Queen)
Lynyrd Skynyrd - Free Bird (epic guitar solo at the end!)
Buckethead - Padmasana (very relaxing song with some great guitar work)
The Stone Roses - I Am The Resurrection (catchy song with a nice bass line)
Like I said these are my current favourites not of all time because I like SO much music...!