The first song that came to my mind was Black Magic Woman, but some of those already mentioned are even better. **Hey Nineteen **and **The Kids Are Alright **would be two of my favorites. Another one that is not only instantly recognizable but seems alter the chemistry of my brain the moment I hear it is Radiohead’s Everything in its Right Place. Amazing song, amazing album.
Four words but only one chord - Bohemian Rhapsody has to be up there somewhere
Johnny B. Goode
Beethoven 5th
Although I think We Will Rock You is probably the answer, I still think my guy is close with School’s Out. You hear that opening riff and there’s not one single doubt to who and what it is.
This thread is kind of strange. I can recognize hundreds of songs after the first few notes. There are very few where the opening chord defines the song though. “Help” has to be the hands down winner for one single opening chord.
Kashmir by Led Zeppelin.
Obviously I meant “A Hard Day’s Night.” And obviously, for me it was. Good night!
‘Help’ by The Fab Four.
I mean, the very first sound you hear is the name of the tune.
Turn it up!
Another vote for Satisfaction.
Radiohead “No Surprises” is clear from the first note.
Or perhaps “thunderstruck” by AC/DC? There’s nothing quite like that initial fiddly bit
“Like a Rolling Stone” - the opening drum crash defines it pretty well.
“Baba O’Riley” - hear that synthesizer, know the song.
“Tubular Bells” - famous for being used in the Exorcist, but an unmistakeable theme.
And back a ways I won a prize from a radio station for identifying “Love Like a Man” from Ten Years After. I was on the phone to them before the opening organ chord was over. Unfortunately the prize was the solo album by Chick Churchill, not something useful like a pizza.
Hmm…I have that album and it took me a good four notes or so before I recognized it. I mean, yes, I know you put the name of the song in the link, but I don’t know the names of all the songs on the album, even though I’ve heard it probably a hundred times or so. No way I would have known it if you just played the first note or two for me. It’s not really that distinctive (in my opinion.)
Me, too! And I always shout along with the “Turn it up!”, too.
Big fun. 
Not quite as well known a song as some of the others but Mountain’s “Mississippi Queen” is pretty distinct.
Once I was sitting with a friend and his wife and we heard a guy hammering in the distance, seven times in quick succession, and I just went “da da da da dum”, cracked them both up.
You just described my night with the twins I met on Craigslist.
mmm
“Let it Be”
“My Sharona”
Nobody here has heard this one, but once you hear the beginning, you won’t associate it with any other song again (NSFW) (but crank it up):
I’ll say it’s snare and hi-hat.
Dancing Queen by ABBA.
Well, most songs that are very well known are immediately recognizable from the first few notes… dunno, I nominate Smells Like Teen Spirit.