I would ask what we mean by "popular music". If you mean popular by Industry terms ("Pop Music") then I would say The Beatles. If we mean popular by terms of most popular artist, then I would say Elvis Presley. Elvis is difficult to call a "pop" artist because a majority of his work was influenced by genres other than pop. Pop music did not really exist in it's true form prior to the mid-60s, And Elvis had been singing for years before that. Elvis' biggest influence was Gospel music. He was, and still is, among the most gifted gospel singers in history. This genre he altered in Churches around Mississippi until he began fusing it with what is now called Country music (then called anything from Bluegrass to Jug Band music, depending on where you were from). The fusion of Gospel and Country gave Elvis a sound all his own, which he later fused even deeper with the Rock and Roll sound of Fats Domino, Chuck Berry, and Big Mama Thornton. Some gents from England had a hit in America later on in the sixties, which ushered in a new age of music. They were called The Tornadoes. (Fooled you, didn't I?) Their instrumental hit Telstar (If you haven't yet heard it, I recommend it) debuted on the charts circa 1962, and told the until then music experts the Americans that the English had a few chords to play as well.
Following the Tornadoes' slight success, other bands from England began crossing the pond, as it were, to find American audiences much more inclined to accept their musical expression than was previously believed. They will almost all tell you that their major influences were Fats Domino, Chuck Berry, and of course, Elvis.
The Beatles were among the first to leave the Cast-Iron shore of the Mersey River in Liverpool to test the Atlantic waters. This led to countless Liverpool acts: Gerry and the Pacemakers, Rory Storm and the Hurricanes, Billy J. Kramer with the Dakotas, to come to popularity in the World. This massive outflux of talent from Liverpool gave rise to the term "The Merseybeat Sound", since some found that Liverpool acts sounded similar.
Later, Berry Gordy of Detroit Michigan created his own record label, Motown (after his hometown of Detroit, the Motor City) and gave birth to countless acts such as The Temptations, The 4 Tops, and the Jackson 5.
Now, here I must digress slightly. I like Michael Jackson's early solo music. The album "Thriller" is, in my opinion, the best album ever put together. Of the 12 songs on that album, 7 made the top ten. Just about every song on that album is solid. "Wanna be Startin' Somethin'", "Beat It", and "Billy Jean", arguably the best pop song in history (possibly for another thread) Puts Thriller on the radar as the best Pop ALBUM of all time, but his other musical endeavors prohibit me from naming Jackson the best Pop Artist of all time. When the REmake of "Smooth Criminal" by Alien Ant Farm is far betteer than Jackson's original, there is a problem.
Best Pop producer of all time? No question, Phil Specter. Hands down, Specter's "wall of sound" defines Pop music for me. "A Christmas Gift to You" is a great example, an album of his artists (The Crystals, The Ronettes, Darlene Love, Bob B. Soxx and the Blue Jeans) singing Holiday staples. "He's A Rebel" By the Crystals also comes to mind.
But this still raises the question of best Pop artist of all time. I go back to the Beatles again, however their range transcended genres as well, to a point that they are almost impossible to classify. Starting out in unquestionable Rock and Roll mode, the five boys from Liverpool (John Lennon, Paul McCartney, George Harrison, Stuart Sutcliffe, and Pete Best) journeyed to Hamburg, Germany in search of fame and fortune, finding there neither. Their Rock and Roll attitudes were perfect, though. With their leather jackets and sneers, they might as well have been Billy Idol's Uncles. Brian Epstein, their late manager, knew that these boys would one day be stars, and forced them to, against their wishes, clean up their acts. They fired their awful drummer, obtaining Ringo Starr as a backup until they could find a good drummer, and Stuart Sutcliffe left to pursue an art career, something which he and Lennon had planned since childhood. Their Rock roots were still there, but they began to make music for the people, and not so much for the money. Thusly, they too generated a new sound, Kind of a Pop-Rock.
Here we reach again, the question of Best Pop Artist of all time. My answer may shock you. The artists I've mentioned above have had one thing in common. They played music for us, we loved it, and they didn't stop until we were nearly sick of it. The Beatles almost left us wanting more, but not completely. Elvis' legacy lives on, but the testimony to his unfortunate musical longevity was evident when the USPS made us vote whether young Elvis should be on a stamp, or old, fat Elvis. Did they really need to ask us?
Along the same lines, we have Michael Jackson. Guilty of molestation charges or not, his image will forever be tainted, and as a result, recent album sales have been lackluster.
By this rule of "Leave 'em laughing when you go", I must give my hand to the late Buddy Holly. Charles Holley from Lubbock Texas was a natural guitar player, a pleasant person, and made some of the best music ever recorded. With the Crickets, he made such hits as "That'll Be the Day", "True Love Ways" (among the more beautiful songs in history), "Not Fade Away", and "Everyday". His death in a plane crash was immortalized with Ritchie Valens and J.P. "The Big Bopper" Richardson. Valens and Richardson would probably have amounted to little more than trivia questions, but had that plane not crashed, Buddy Holly and the Crickets would have been as big as the Beatles, if not bigger.
My answer: Buddy Holly and the Crickets