I love 'em both, but "One Thing" for sure.
It's the quintessential definition of timeless pop, accessible and their most "beatle-esque' song.
"What Makes You Beautiful" is cute, but I think it's limited when targeting an audience or certain demographic.
"One Thing" can pretty much be reached with a broader audience.
Not to mention, it's one of the few songs where the entire group has a solo.