Who was/is the most significant?
And, to expand that question a wee bit: Who was it that had the most important place in the evolution of popular music?
Performer(s), writers, popularizers, any role you think was important.
For example, Elvis may not have been an innovator, but he did play a part in bringing Rock and Roll to the attention of white America, and the rest of the world too.
Marconi was not a musician at all, but without radio, Big Band Music as a major musical chapter would never have happened.
Don Jorges Julio de Pasqual Fretboard, may not be a name on anyone’s list, but if he hadn’t invented the guitar, where would we be? 
So, to start it off, here’s few of mine, in no particular order.
(I’ve said whom, and why.)
Bob Dylan. His style of “folk music” was not the first, nor arguable the best, but he was critically important in making it a pop “style”.
The Sex Pistols. By the mid/late 70’s, pop had become ossified, but the sex pistols position in punk and it’s revolution of pop was very important I believe.
John Mayall. Not very famous himself, but his band’s role as the nursery of British R&B cannot be denied.
Benny Goodman. He didn’t invent swing, but he was the one who got everyone dancing to it.
and so on…many more.
OK, your turn. 
Ali
Easy, Bach. Without Bach we would never have the concepts of western harmony we have now, the tempered instruments we use now, the scales we use, etc.
Les Paul for pioneering multitrack recording.
The Beatles for showing us what to do with those multitrack capabilities.
Personally, I’d put the Ramones ahead of the Sex Pistols for carrying the flag for punk - I heard them (and heard of them) way before the Pistols.
Jim Marshall for designing and building the ultimate rock amps, and everybody at Gibson and Fender in the '50s for designing and building all those awesome guitars.
Jeff Beck for showing us that it’s okay to think “outside the box” musically.
Ringo Starr for proving that taking a straight 4/4 beat and sitting in the pocket and whipping the fire out of it kicks just as much ass as anybody who throws fills in all over the place.
Very nice topic, Ali !
I think Queen was groundbreaking.
In the Christian Music arena I think that Don Fransisco was quite instrumental (!) in the ‘evolution’ of Contemporary Christian Music…
The Everly Brothers started something that (in my opinion) Simon & Garfunkel took wider into the world.
Remember Kimera ? The 1st opera singer i remember to enter main stream music.
Andrea Bucelli (to me) was the 1st opera male singer that made me go out and buy a CD of a ‘great tenor’. He opened up a new ‘listenable’ area to the opera/classic world for average guitar lovin’ folks like me.
Montserrat Caballe and Freddie Mercury - when rock and opera met…
Metallica S&M - When metal and classic met. That was really good.
Micheal Caman - He did some very nice stuff for Pink Floyd, metallica and numurous others…
Nice subject!
Johann Strauss for being the first composer to make me aware of classical music.
Ritchie Blackmore and Jon Lord for showing me that Heavy rock could be clever and full of prodigious talent
Paul Rodgers for showing me that Rock could have soul
Status Quo for proving just how much fun live music could be
Stuart Adamson for writing from the heart
There are many others but it’s a start!!
Great topic Ali!
Mozart for being Mozart
Robert Johnson (What Else Can I Say)
Les Paul for Les Pauls
The Beatles for Excellance
Jimi for not buying the accordian?
?
I vote for Edison, Flemming, and De Forest, for the development of valve/vacuum tube technology - and recording technology, in Edison’s case. Is “popular” music music that is distributed by electronic means? What’s the relationship between popular music and electronic distribution?
Here’s a small page with summaries: http://www.ee.umd.edu/~taylor/Electrons3.htm
King Crimson
Robert Fripp
Ian McDonald
Mel Collins
Greg Lake
Bill Bruford
David Cross
John Wetton
Adrian Belew
Tony Levin
Trey Gunn
Pat Mastelotto
Hey Jason - how’s the studio going?
John Lennon & Paul McCartney
Bob Dylan
Elvis
Jimi Hendrix
Led Zepplin
CSN&Y
Joni Mitchell
The Buggles!
Easy, Bach. Without Bach we would never have the concepts of western harmony we have now, the tempered instruments we use now, the scales we use, etc.
I think you nailed the beginning there, Chip... and just about every other response nailed an element. Popular music is a melange... an amalgam. I've not yet seen nods to Berry Gordie and the entire Motown scene, the great Blues and C/W players who elevated the guitar to its spot in pop music, Ray Charles, who carried the sounds of black spirituals into the mainstream... you'd even have to toss Segovia in there, methinks.
It's an evolution. How can you point to any one branching point or step forward and call it the most important? Take any one out, and the whole chain fails - or ends up in a completely different place.
I think you nailed the beginning there, Chip... and just about every other response nailed an element. Popular music is a melange... an amalgam. I've not yet seen nods to Berry Gordie and the entire Motown scene, the great Blues and C/W players who elevated the guitar to its spot in pop music, Ray Charles, who carried the sounds of black spirituals into the mainstream... you'd even have to toss Segovia in there, methinks.
It's an evolution. How can you point to any one branching point or step forward and call it the most important? Take any one out, and the whole chain fails - or ends up in a completely different place.
Yup, Bach is the man. Everyone else listed are just blips on the radar. You don’t have to like Bach (you should ), but you have to give the man credit for establishing the entire frame work through which western music has evolved int he past 400 years. Before that, everything was either modal or had no meter. Try finding a time signature in Gregorian chant or a diatonic scale in renaissance/eastern music. Good luck. No Bach, no bass, no scales, no tempered tuning, no harmony…
Funny, I’ve always heard Bach described as a synthesizer of other trends, not an innovator.
edit: I noticed today taht I had typed “Bosh” - oops… 
I wouldn’t say innovator, I would say “popularizer” if that is a word. Think Henry Ford. He didn’t invent the automobile, but he just popularized the concepts and designs that we still use today. Or in math think Euclid. He was the first to really combine all the ideas of time into one place.
How bout prehistoric man for inventing percussion?